The Band T-Shirt Question

So, I recently have learned theres two rules for concerts.
1. Do NOT wear a bands t-shirt to their concert.
2. Do NOT listen to a bands music to their concert.
So, okay. For number one I don't get it because if you're going to your favorite band's concert, why NOT wear their t-shirt. If you love them why can't you show it? Also what about for example, if I'm going to the All Time Low concert which is sponsered by GK can I wear a GK shirt or a JAGK shirt? Or does that too fall under the band t-shirt category?
Number two, why can't I? Does this only apply to poseurs who want to learn all their songs within a few minutes? Using the ATL example again, I've been a fan since Put Up or Shut Up came out so do I get a reprieve? I know all the songs by heart, what's in parentheses on SWIR, and I can list all the cities in order from Hello Brooklyn, so can I listen to it on the way up?
So since I'm sure there's someone whos been to a lot more concerts than me out there who probably could tell me, I'm calling that person out. If you know the answer to my questions about the two rules, just comment. Thanks.