Why does it matter??? They are run by some of the better managers in the business, I am sure they aren't taking too much risk. It would be one thing if it was obvious they were gambling, if many lines were off both ways, but they almost always offer a better price on the dog so you just have to assume they prevent getting hit too hard by their chalk loving players.
I doubt they lay off their action much, they really don't need it. Enough decent players are out there to take the dog when it gets out of line, if you notice they are usually only a hook off the prevailing line so not enough to get much in the way of scalps.