petermc It's absolutely a legal issue in that card companies don't want to be sued into oblivion if they inadvertently process 'obscene' content.
The reason GoT can show incest and adult sites can't comes down to the aforementioned 'obscenity' and 'artistic merit'. GoT is not an incest porn series. There is an actual non-sexual plot that takes up the bulk of each episodes. Your average person is not watching GoT for the bits of incest sex here and there (and really, there isn't much outside of the very first episode). Conversely, your average person watches adult content for a very certain reason which may or may not have enough artistic merit to keep it from being considered 'obscene'. This is the fine line card companies need to take when accepting our business.
Again,. bankers, card companies, etc. would LOVE to process everything legal and not have to worry about it, but the law of the land plus America's litigious nature makes this difficult. So again, if you want to see more currently proscribed content, lamenting on this forum will not help. You will need to contact politicians or consider running for office.