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.
Showing before tax prices (almost) makes sense in the US where every other street has it's own local tax rates. But Canada only has federal and provincial taxes so I suspect it's just to make the price look better - especially if you are comparison shopping with the US
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.