Now, I am just a disinterested observer here (with investments in Bitcoin and all major 2.0 cryptos) so I don't speak for anybody but myself here; but it seems to me that Counterparty has Bitcoin over a barrel somewhat, especially with respect to the unspent outputs, which would be quite difficult to selectively block on the Bitcoin end.  It would quickly turn into a cat and mouse game.  Bitcoin could implement code to detect and block XCP transactions, but XCP changes a few parameters and re-launches a week later to get around it, which causes Bitcoin to respond, which causes a new change in XCP, etc. ad nauseum.  I don't think that anybody wants that.