So did they give you a citation or just a verbal warning? It seems to me that police that just give you verbal warnings are bored, curious as to what you are doing, and want to give you the impression that they have the entire law contained inside their head for reference, when in fact they don't. Then when they investigate further to find that you actually haven't done anything illegal they reprimand you as if you are still the bad person and send you on your way with a hand slap.

If they didn't tell you that you could not ride the thing legally then I see no reason that you shouldn't as long as you don't get carried away and ride it foolishly - that would guarantee to draw attention to these bikes.