In Bridge, what are "proper" responses to a "strong" 2 club bid?
If that's the system you've agreed with partner - that the only valid responses to 2C are 2D or 2NT - then it's fair enough to raise an eyebrow if they start improvising.

Personally, and bear in mind that I don't play Standard American myself, but a pretty loose take on Acol: I don't understand why you would want to restrict responses so strictly.  Fair enough, you need to have a negative 2D bid, and to understand between you what that implies.  Beyond that, the more expressive you can make your system, the better.  What's the point of a 2NT "generic positive" bid if it just forces opener to ask more questions, at the next level of bidding, to determine the actual best contract to be in?

If my partner has enough points not to have to make a 2D bid, I'm delighted to hear about hearts or spades being his longest suit at the 2 level, even if that suit is only 5 cards or whatever.  Naturally, if partner wants to talk about a minor suit, at the 3 level, they'd better have more than a 5 card suit, to be on the safe side...