The War on Bongs

The Drug Policy Alliance reposts a new piece in Rolling Stone on the recent crusade against bongs. Question for the legal-eagles present: does it violate the Constitution's prohibition on ex post facto laws to begin enforcing existing laws against conduct that previous interpretation had given business owners reason to believe was legal? That is, if it had always been the case that head shop owners were left alone under drug paraphernalia laws, provided they marked their pipes as "for tobacco use only," can the government arbitrarily decide that this is no longer good enough and begin breaking down doors?