I have no idea how much vendors use XMR but unfortunately we cannot prevent the junkies and criminals using the best alt.
What is considered 'criminal behaviour' is very very relative in time and space, depends on who's in charge, societal climate etc etc... And yes, these people, whose behaviour has been outlawed will seek the best way to hide said behaviour... This is not something which anyone should seek to prevent, at least not by diminishing anonimity/privacy and thus fungibility in a currency
Criminal = outlawed.
If you live in Somalia which has no law and order there is obviously many things possible but if we look at the majority of countries, most of them has outlawed drugs and certain type of pornography for instance.
The criminal activity has partially ruined the reputation of bitcoin (many still think it is a currency of criminals) so it may not be good for the reputation in the beginning. Therefore, being accepted by the D.N.M. is kinda double edged sword: on the one hand it is nice that XMR has some real use case, on the other hand, it might create reputation to the XMR of being the currency of the bad guys.