Even the Gmod and Source Film Maker communities are more respectful than the SRB2 workshop as they will remove a re-upload if the original author wishes, but not the SRB2 Workshop. Speaking from experience on this one.
Studios have the right to set rules when it comes to fans making use of their IP's. Sega happens to be one of the friendlier and less restrictive studios at least when it comes to the Sonic roster.
This is the reason why you do not see any highly developed fangames based on Nintendo franchises. Nintendo though recently 'endorsed' fan game creation, but only if you purchase and use their limited game making toy (Game Builder Garage) for the Switch which will not allow a quality that gets close to the official titles.
Except Sega has never explicitly stated anything of the sort anywhere.
Their position isn't "we have allowed this and anyone can do whatever they want", it's "we don't care and we won't do anything about it unless we have to".
Contrary to popular belief, Sega has taken down Sonic fan projects and other unlicensed projects made with their IPs before, Sonic Eclipse Online and a fan remake of Streets of Rage being major examples of this.