Via a CBC news article regarding prostitution laws in Canada
Prostitution is legal in Canada, but communicating for the purpose of prostitution is not.Huh?
(PS: I do not have any real difficulities at all with the legalization of prostitution in Canada, or with setting up brothels for that matter)

No riddle. It is too hard to define what prostitution is -getting a fur coat from an admirer? marrying for money? whatever.
So soliciting becomes the crime since it is easily defined.