A probationary period is not a way to pay a guy less, but as stated above, a way to weed out someone who shouldn't have made it there to begin with, without the major expense (for both the union and) a corporation would have to otherwise spend in attorney fees, to get rid of someone who doesn't fit in. Probationary pilots do have the vast majority of negotiated rights, under whatever labor agreement they work, however, for the first year they are still an at-will employee, meaning they can be let go for no cause whatsoever.