Now imagine you are boosted to 90, go through the WoD content and get to level 100, gear up and get ready for raiding and then realize you need to redo all old content (for gatherers) or make enough money to buy mats at AH to level professions, before you can begin raiding. That would essentially defeat the purpose of the level boost.
Or as others have said, the profession leveling system does not keep pace with the rapid experience gains we now get, so until that system gets revamped, I don't see a problem with a profession bump along with the free bump to 90.
As far as pay to win is concerned....I don't see this as game breaking in any way. It just makes it easier to level that new alchemist, but does not
give anyone a meaningful advantage for end game content. That is the only measure of pay to win that I care about and as of yet, it is not an issue.