Well this is interesting. Back in "my" day (being old and all) it was amateurs vs professionals and the restraint of trade was simple. If you earned money from cycling you got barred from the Olympics; and if you joined the local cycling club and it happened to be a pro club you may as well have joined a different universe. But that's all gone now and "amateurs" don't really exist, instead elite athletes are professionals whether they like it or not, and it's "all in". Unless you run a significant yet smaller road race or tour, in which case the cycling world's governing body, the UCI, can impose its will and stop top-level teams from competing. I guess there's good and bad in that rule, but I do like Chris Horner's comment:
On Monday, Horner said the enforcement of the UCI rule was "wrong." "It’s a pro race, you should be allowed to race your bike. If we are skipping ProTour races to do a non-ProTour event, then it makes sense. But you should never, never, never just not allow a rider to race his bike. ... every man should be afforded the right to work."
The opposing view may be that a ProTour team will simply scoop up all the winnings, leaving the other guys (also just trying to do their job) picking up the leftovers.
