Personally, I blame this on the shift in mentality away from "balance is found during a game session" to "balance is found in the design of the rules" that occurred (for D&D at least) when Third Edition was released.
This created a presumption that everything put out by the first-party publishers (WotC, for D&D) was balanced unless shown otherwise. In other words, players would simply take it for granted that everything WotC publishes is allowable in play - there's no need for a discussion about if they can use something, they just show up with a new book and expect that they can use it, being honestly surprised if the DM objects. Obviously, that's a generalization, and so won't apply to all players in all cases, but the general sentiment is there.
Essentially, this creates a train of thought that goes like so: "If you (implicitly) acknowledge that this material is balanced, then how can you have any non-personal reason for disallowing it at the game table? After all, if something is balanced then it by definition won't be disruptive. Ergo, this is you either pulling a power trip, or making your personal issues into my problem."
I don't agree with that line of thinking, but I've run into it more than once.
This puts the GM on the defensive, as they now have to swim upstream to say why this "balanced-ergo-non-disruptive" material is being disallowed anyway.
One possible answer that some GMs use is "this is being disallowed as an 'appropriateness of the setting' thing." Unfortunately, the issue of "disallowed for in-game reasons, rather than balance issues" opens up its own can of worms.
Basically, this one runs afoul of what some people's ideas of "PC exceptionalism" means. Some people don't care that their characters don't fit the tone of the campaign world, because the nature of PCs is to be characters who break the mold. As such, it's not a big deal if their character is completely at odds with the in-game assumptions of the setting, since they think that's what they're supposed to be doing anyway.
As with the above stance on balance, I don't care for this idea, but it's not an unpopular one that I've seen.