[Speculation:]
Perhaps they've run the numbers and can figure out that they make enough money from enterprise clients and will make enough more money from the 'marketplace' being a channel for selling github integrations and addons to cover this cost of not trying to monetize through supporting teams.
It also moves a large base from 'customer' with needed support to free users which don't need the same level of support.
Do they? Unless you're on GitHub Enterprise, migrating is just moving your repos over the weekend, setting up new webhooks, emailing everyone a command to switch their upstream URL, and hoping the new workflow works for you. For teams of <100, this it one of the easier transitions to make.