Forgive me, as I am not up on all the details, but isn't that standard operating procedure for pro sponsorship in a sport? For example, Dale Earnhardt Jr., as per his contract, is not allowed to be publicly seen or photographed drinking any other beer besides Budweiser. He is allowed to drink any beer her wants I as long as it is done privately, but not to be seen tacitly endorsing a competing product to the company that is actually paying him. Doing so would be a violation of his contract. Soft drink companies do the same thing.
I could be missing something, but it seems to me if riot is sponsoring you to endorse LoL it is part of a standard commercial contract that you not publicly endorse a competing product. Now, there is plenty of negotiation room in what a competing product actually is and Riot's definition might be too broad, but I would say anything that falls into the MOBA category is certainly valid.