It can be paired with bitcoin, and then all transactions are for that product, not the bitcoins. Could be a e-card or something like that.
Are you saying if someone sold you an iPhone and an ounce of weed for $100, they are not a drug dealer, just a lowly handset merchant?
Edit: I think drug dealers just be drug dealers and not worry about selling eg Amazon vouchers as well.
Bitcoin is not drugs and they are not illegal. This is only for the future event that FINCEN decides all transaction in bitcoins should be money service registered, when trading for dollars.