Well for one, Major teams play a lot more games because they make it to Sunday most weekend so their players are going to have more total walks during the season in general.
But you have to go by walk percentage or walks per plate appearance to see who walks the most. And those are not major players.
Here is last years leaders in walk percentage. This myth is right up there with the belief that Major teams play lesser opponents and E teams every Friday night. Definitely not true and easily proven by the stats.