Not just that, but the DM should be in control of what rules apply in his/her campaign. It's awkward when it becomes the default presumption that players can import who rule-sets, and it puts huge extra workload on the DM (they have to keep up with each and every new book, whereas with campaign-centred books they don't need to read them if they're not interested in that play style or world background).
Sure, the DM can always say "no", but this creates a very awkward situation**. A player excitedly rushes in with a new book, saying "I can't wait to do this!" and the DM says "not in my game you're not". To which the player thinks "waah - you swine - I just spend £30 on this!". When actually, this wouldn't have happened if the hateful book hadn't been released in the first place.
Thankfully, the first "splat" release does exactly that, by keeping it specific to a campaign. I won't be running the campaign, so I don't even have to think about the damn thing, let alone read it, care what's in it, or have to rule on whether we're using it in my game.
And don't get me started on the "you have to buy this to keep your character competitive" aspect of it (I've never liked the damn things ever since TSR explicitly advertised them this way

).
TL;DR: Let's keep supplementary rules firmly in the province of the DM. If the DM doesn't explicitly go out of their way to choose them, they shouldn't be a player option. By all means release cool new options, but let's frame them as DM books, not things the players feel entitled to bring to the table.
*that's not to say that the DM shouldn't listen to player tastes, of course
**I am English, and avoiding awkward situations must be done, at all costs