Yikes, lots of negatives in one sentence makes it hard for me to parse. Lemme try:
why some players don't think "I just don't like X" isn't a valid reason for a GM not to allow something
They DON'T think "I DON'T like" is NOT a reason to NOT allow something. That's four, so I think they cancel out as two pairs of double negatives?
So the question is Why do some players think I like X is a valid reason for a GM to allow something?
Well, it is a valid reason. It might not be one that the GM accepts, but "I like something" is as good a reason as any to include something; and "I don't like something" is as good a reason as any not to do so.
It depends on the context, really. Your "metals" example sounds overly picky to me; I'd laugh and move on. If they really pushed it, then I'd be questioning what was wrong with them; maybe rename it to "Xthranite" to keep them happy or something, but that would just me being sarcastic.
If it was something more substantial - "I don't like horror themed games" - then that's a discussion to be had. The GM shouldn't run a horror-themed game for a group of people who dont' like horror-themed games. I mean, what's the point? We're all there to play a game we like.
But no, the metals example is just a player being a bit of an ass. I'd not be worrying about the game here, I'd be worrying about the ability of the person to interact with others in a mature social fashion.