It's not so much that outcalls are legal and incalls are illegal. Incalls at a fixed location where the business of prostitution occurs will lead to "Bawdy House" charges for people "Found In" that location under the Criminal Code of Canada. When an SP comes to your house, a Bawdy House charge is not relevant. This is what makes outcalls legal.
However, the issue of hotels/motels is kind of a grey area. Think of it this way - if a hotel/motel attracts a lot of guests that "conduct business" at the locaiton, so to speak, then it could run the risk of being charged as a Bawdy House (a place where the business of prostitution occurs), and the owners of the hotel could be charged as "Living Off The Avails" of prostitution through the rent being charged.
While I think it is exceptionally unlikely that a hotel/motel would ever have a charge like this laid against it (or have it actually turn into a conviction in court), it is a reason why some owners may want to keep a lid of the activity. Of course, there is also the fact that hotels/motels simply don't want this activity in their business.
I think the hotels/motels that would have to worry about this are the ones that are in high traffic SW areas, and even still it is rare. In the MP section, there was some talk about police hassling owners of plazas that had high concentrations of MP's in them. However, I don't think that the owners were charged with anything - but if you're a hotel that cares about your reputation, they probably don't even want the hint of any hassle.