Unfortunately I think this is much easier said than done. No single store is going to want to make this change, because it'll make their prices look higher than the competitors'. It'd require legislation, (and even that'd likely be state-by-state legislation).

It also means a company wouldn't be able to advertise a single price for a product nationwide, since sales tax rates vary by state (and many times even within a state).

Also worth noting that Canada also doesn't include sales taxes.