"Rulings, not rules" does not grant DM's any freedom they didn't already have. Every. Single. Edition. Of this game, has had a large swathe of DM's (if not, eventually, all of them) who have made their own house rules to modify the game.
The DM never had their power taken away by the rules of the game. Never. Not once. 5e doesn't empower the DM any more than any other edition of the game ever has.
It's not a question of the referee being able to or not. It's a question of normalizing things. It's a question of the books saying the referee is able to...so that when the players inevitably freak out, the referee can point to that line or paragraph in the book. It never seems to ease anyone's trepidation, but it helps...sort of. It's about the players reading the book and seeing that the game expects the referee to make house rules. Instead of the book never mentioning it and the players being surprised when the referee mentions house rules.
You might say "well, other editions have had complex, interwoven rules that made it difficult to change any one thing without causing other problems". To which I reply, yes, and?
Making things harder to accomplish does remove people's ability to do the thing. That's basically the point of laws. Make it really hard to get away with doing something and you've effectively reduced the instances of it happening. Make house ruling your game really hard to do and you effectively reduce the instances of it happening.
To make good house rules, one must understand the purpose of the rules they are changing in the first place.
Exactly. Hence more complex systems are harder to house rule, hence fewer referees are willing to house rules those systems, and since not every referee is a professional game designer, when they house rule complex games, they tend to do so badly...which gives the players a negative reaction to house rules and an aversion to house rules in future...which reinforces the notion that referee's shouldn't house rule complex systems...thus effectively removing the referee's ability to do so. Because a referee who freely house rules a complex game in a community that actively discourages house rules will inevitably find themselves staring across an empty table.
5e does not make this task any easier.
It does by having simpler mechanics that are easier to house rule and the text supports the notion of house rules, whereas other more complicated iterations of D&D with no explicit mention of house rules curtail referees from house ruling things. When you talk about these things in the books themselves, you center that as a normal part of the game. When you don't talk about these things in the books themselves, you erase or remove that as a normal part of the game.
Take for example 5E compared to 4E. The phrase house rules appear on page 6 of the 5E PHB.
"you should check with your DM about any house rules that will affect your play of the game."
It does not appear
at all in the 4E PHB. It only appears in the 4E DMG, and the section there is one page that is mostly about admonishing the referee to not house rule things without knowing exactly what they're doing. In the 4E PHB the role of DM is described in four main parts, the one relevant to this discussion is:
"Referee: When it’s not clear what ought to happen next, the DM decides how to apply the rules and adjudicate the story."
Note how it doesn't mention changing any rules. Only that "the DM decides how to apply the rules." The DM has discretion in applying the rules, not in altering them. Further note how it's not a general statement of "the DM decides how to apply the rules," rather a quite limited "When it's not clear what ought to happen next..."
Compare that to page 4 of the 5E DMG:
"And as a referee, the DM interprets the rules and decides when to abide by them and when to change them."
and
"The D&D rules help you and the other players have a good time, but the rules aren't in charge. You're the DM, and you are in charge of the game."
Normalizing things makes them easier. Ignoring or actively erasing things makes them harder.
Again, yes, referees always can and always have house ruled games. It's a question of the expectations of the players. When the books do not mention house rules or the referee having the authority to change the rules, the players expect that to not happen. When the books do mention house rules or the referee having the authority to change the rules, the players expect that to happen.
Doesn't mean that given players will accept specific house rules from any given referee. Just that it's a normal part of the game.
Does anyone really understand why, if I cast a spell as a bonus action, I cannot cast another leveled spell with my action on my turn? When an Eldritch Knight can cast two leveled spells via Action Surge and cast Shield without incident?
Balance. The eldritch knight has fewer spell slots and doesn't have access to the same incredibly powerful spells full casters get. So it's less of a problem that they get to cast more spells.
There are many rules like this, not even picking on the optional ones, where playing the game without intending to make a single house rule, but completely by the rules as written, eventually becomes a Sisyphean task!
The only thing that is empowered by this is my headaches.
Exactly. "Rulings not rules" and "it's your game, house rule it" are a get out of jail free card for the designers. They don't have to work as hard making a well-designed game that interconnects perfectly and just works when they can simply do a good enough job and say, "don't like a rule, change it."