If someone buys the L lens for the M, the cost of adapter is a small change. Why not?? On the other hand, the M mount lenses has already cover from 11mm to 200mm. That is more than enough for most people. One of the idea od M is to keep it small. Put a big L lens on it goes the opposite way. I have a 17-40 L, I have never consider mounting it on the M.
That's the problem. The purpose of mirrorless is to take stellar pictures in a much thinner body, hopefully with smaller lenses as well (but the sensor size does have something to say about that).
But Canon knew that the first people in line to buy an EOS-M would be mirrorless devotees -- it would be existing Canon guys with lots of glass who want a very small 2nd or 3rd body. That (plus the lack of native EF-M lenses) is why the adapter was available on day one.
So -- at least with Canon -- you have two camps of EOS-M owners. They don't see eye to eye too often re: what should be mounted on it.