By telling them "No tieflings" they also have that same opportunity.
You somehow mistake that something is removed other than the tieflings in that case.
There is no "Mother May I" with ANYTHING banned. The fact it is banned, forbidden, disallowed, is the problem you have and issue you are having.
The automatic answer is NO, that is what banning is.
Lets call this "why" question as similar to threadcrapping. There is a line drawn that something isn't allowed in both cases. The rule was set, and if you come to that then you accept the rule.
You don't go to a 9-ball tournament to play 8-ball. You try playing 9-ball in an 8-ball tournament, and you are soon going ot be screwing up.
Here is a why for you: Why would someone be going to play for the sole purpose of confrontation? If you don't accept a game with "no tieflings" then why even go there?
The players requested and being sought were ones agreeing to "no tieflings". When a player then shows up wanting to dispute that, they have already violated a part of that "social contract" and shown they have no interest in communication, because they started out by not paying attention and not following the rules set forth by the DM.
This behavior 99.998% of the time carries over to being a disruptive player.
Seriously, why would someone go to a game set up with the rule of "no tieflings" be there with an attempt to play a tiefling, or even care?
The DMs job is to make the game work. Not to let just every Tom, Dick, and Harry off the street try to tell everyone else how to play. If there are other people that agree with this rule of "no teiflings" except this one, then you already see this one person asking doesn't have views aligned with the group.
If you already knew what to expect and then went counter to that, you are only there to BE disruptive to the others.
The school example was given, someone going to heckle a comedian, etc.... These are all the same type of disruptive people. There is no reason a DM or other player should have to put up with that.
You don't have to agre with it, and the fact that you take issue with it would make me ask you this:
Would you come to mine or his games saying "no tiefligns" and them try question that?
If so, then why did you come in the first place? Just to question why tieflings aren't allowed?
The person raising the question when the ground rules were set, should never have come if they didnt agree with those rules.