OK, lets try a hypothetical.
No. Thank you though.
Because they don't know you're altering the results.
You think they don't. And for other DMs that are not you, sometimes the DM might be right, but more than many DMs think, the players get a feeling or know that you are.
What about those of us that would rather the DM run the game instead of dice?
Use a diceless system. Why use a system that focuses on die rolls for resolution when you are not going to rely upon the die rolls?
You've determined a random encounter is appropriate, you've rolled on a chart, and come up with a result that will definitely result in a TPK, and that makes no sense to the story.
In all of these hypotheticals you are missing so many steps.
First, the DM has made a mistake by rolling on that table.
Second, the players have made a mistake by choosing to engage in combat.
Three, the DM has once again made a mistake by choosing to have the NPCs engage the party in combat.
Fourth, the players have again made a mistake by staying engaged in combat
Fifth, the DM has made a mistake by demanding the results be a TPK.
There are so many things one can do with an "impossible" encounter rather than try to fight and lose.
Sorry guys, I know you're 1st level, but the dice say a Balor appears and kills you all.
No it doesn't. The die roll says their is a balor nearby. The party is beneath it, why would it waste it's time? Do you go stamping on and killing every ant you cross paths with? I don't, and neither does the balor need to. Perhaps it picks up the ant and looks at it. Maybe if the ant bite it then the balor will flick it away and curse. Perhaps the balor is seen at a distance. Or is in the middle of a nap, or a ritual, or... you have an imagination, use it.
On these forums there is subset of people who as players are terribly invested in how the GM runs the game behind the curtains and want to police the GMing practices.
Or, perhaps it is a sub-set of people who have experienced DMs doing things that ruined a campaign (or three) and are willing to share their experience so other DMs can be aware of the potential for ruining their own campaigns with things that they think are harmless?
But I doubt a GM altering a die roll couple of times in campaign would be noticeable or affect the play experience.
Probably. But with all internet debates, moderation is never what is discussed

Because of course if you are willing to fudge, then it must be that you will fudge every roll
G
Do you proceed with the randomly generated encounter, or ignore the roll and select a more appropriate one, that doesn't result in a TPK?
Which is it?
You persist with these binary examples. Free yourself, encounters do not need to have binary outcomings. (see my comments above)
OK, you select a 'random' encounter for your PCs instead of rolling, and throw it at them. A bunch of mindless Zombies shamble into the room, as a portcullis drops, trapping the PCs in the room
Again, pointless binary hypotheticals. Let's see how many mistakes the DM has made in your example;
1) rolled on a table with a result they don't want
2) made the NPCs attack the party (why can't the zombies be on a walk somewhere like lemmings? It could lead to a whole new story line with a cult and a necromancer and...)
3) the DM caused the portcullis to drop
There are 3 things right their the DM can do without fudging and completely change your predetermined "certain" outcome.