Interestingly enough sportsbet.io is legit in the eyes of law and it can be proven very easily. For example right now there is a jersey partnership with Watford, there was something with a Brazilian team as well but lets say Brazil is a shady country (it is) so lets leave that behind for a second and only focus on Watford one.
When you partner up with Watford you have to provide some sort of contract and have an agreement with the team, when you do that there are tons of stuff that FA has to check about you and if you are legit then they will agree that Watford is capable of going out with that jersey.
Just to give an example, if Watford was agreeing with a cocaine dealer to put his name on the jersey FA would have declined the offer (and would probably look at watford differently
) but when its sportsbet.io they agreed, which goes to show you that sportsbet.io has passed the legality from a top football ossification. Hell if Watford could make it to Europe, that is global acceptance by FIFA which would be basically saying "sportsbet.io is as legit as it gets".