In the same way you ban drugs.  You cant actually stop people using/distributing drugs, but you can make it illegal.

So whilst an individual might take the risk, a legit company would not.  Google would have to block apps in the UK play store that don't comply.

Unfortunately most people haven't got a clue, and anything that will "stop terrorists" must be a good thing.  In reality, companies will follow the profits, even if that means having to comply.