I've never seen any sign forbiding bikes on the canal side pathway from BW to MGM, other than the surrey bikes. There is a sign that says no surrey bikes beyond this point. There is also a sign that restricts access to the service road (not the path). There is another sign that limits surrey bikes on the last section of the pathway to Epcot.
Consider that the rental bikes are located at the BW Community Hall that is itself located along the canal pathway, just past Luna Park. How do you get a bike to and from Community Hall without using the path along the canal? They don't want you on the service roadway.
I think that the rollerblade prohibitiion is different. I was told that there is a Florida law that forbids rollerblading on public walkways. Actually, the path along the canal is much wider than the typical trails along abandoned railway right of way here in the midwest. These are frequently shared by walkers, joggers, bicyclists, and even equestrians. Proper cycling practice is to give an audible warning as you approach from behind ("passing on your left") and then slow down to make sure the party you are approaching doesn't step into your path. I am continually amazed at the people who turn around to get a good look at you, then step to their right to "get out of your way". Of course, after they've turned themselves around, their right is now your left so they are stepping right into your path.
Ralph