You can view the collector card menu on the main title screen. I believe almost all of the slots you can see on there can currently be unlocked; maybe not quite all of them but if not then most.
As for Easter eggs - the game keeps track of them but I'm a bit slack with that. There aren't very many, though.
So the reason they do it is, particularly with something stylised in its design (like my characters) it's seen as a grey area and there are some creators who try to exploit it, so they just err on the strict side. I should be clear that I never got in any trouble for Fumi's design, but I'm trying to just be safe enough to not get suspended or anything.
Otherwise I probably would do more directly Fumi style characters, because I enjoy them too.