Generally things like this are very much a grey area. Riot employees have to be very careful what they say is allowed because otherwise it can come back and bite them in some future unrelated legal case. There is a whole 'grey-market' with selling fan made tshirts, or fan made art for any IP of any company. Some of it can probably be covered under "Fair Use" (for example, a parody of any Intellectual Property is completely fine with or without the consent of the owner - this is how Weird Al can use the same music, or how The Daily show can use copyrighted clips) but how far this protects is a very blurry line. Whole sites out there exist just to sell fan made tshirts of all kinds of IP - especially combinations of IP (my sister has a Grinch - Dr. Who mashup shirt) that I am sure the owners of that IP would never willingly allow, but due so largely under a claim of "Fair Use" that gives them a certain immunity.
Also there are some weirdness in some IP laws. For example in trademark law, companies are forced to be super protective. Say some random small-time Joe sells some tshirts with Coca-cola trademarks on it and doesn't stop when asked by Coca-cola. Coca-cola is almost forced to sue even if he is doing the company little harm. It isn't because they are being mean, but because if they don't that in and of itself can be used in any later court case as a reason that 'they don't care about their trademark' and can actually result in them loosing the trademark altogether.
Note that Trademark law, Copyright law, and Patent law are all COMPLETELY different... and very complicated.
In short... it is called a 'legal grey area' for a reason. No one really knows until a specific case is decided in court. All we can do is guess lol. I would recommend caution.
~David
