So Mom and Grandpa, and then even Dad, all worked in Starfleet, and that's how David Marcus hated Starfleet?
I've been thinking this matter over. Keep in mind, in TWOK Carol was a civilian. It's possible she was in Starfleet in the Prime timeline, but when she left it wasn't under the best conditions. Her bitterness about Starfleet obviously rubbed off on David which is why he hates Starfleet.
Of course even today you have people who serve in the military but their children develop a dislike of it. No reason that won't change in the future.