There's no law against it (some places do this, most notably street carts, etc.), but if you were a cafe owner would you really want to put 15% higher prices on your menus? People would look at the numbers and leave, rather than noticing the 'taxes included' and having to do math.

Until it's a legislated requirement (and boy do I wish it were), we're probably not going to see it very often other than with fungible cash-only vendors.

I thought they don't include tax so they don't have to change it every time the tax rate change.