This likely depends heavily on the terms & conditions of the ticket issuer at the time of purchase.
It's not obvious to me that your credit card processor should be forced into dispute resolution in lieu of small claims courts... but if they are being asked to serve that function, then it's reasonable to request evidence from the merchant before rendering a decision based on your accusation.
You may very well be right: maybe they're ripping you off and maybe the ToS are unenforceable.
What I'm saying: adjucating these situations is the purview of a justice system (ie courts). If you were ripped off, you can always go after the party that caused damages (ticket issuer). If you want a "short cut" to "justice" provided by a credit card processor, well then they get to decide when & how -- and I don't think it's unreasonable for them to collect "evidence" from both parties to make more-informed decisions. And again, if you dislike their rulings you always have small claims court at your disposal.
It's not obvious to me that your credit card processor should be forced into dispute resolution in lieu of small claims courts... but if they are being asked to serve that function, then it's reasonable to request evidence from the merchant before rendering a decision based on your accusation.