A) they don't double dip on the marketplace, developers (or publishers) get the steam tax on all items sold there.
B) maybe, but they provide unlimited nonrate capped downloads for eternity for your game, including free unlimited downloads and uploads of workshop items. Bandwidth isn't cheap at scale, you could spend >50% of your income as a small dev on distribution, or for 30% you get everything steam provides for eternity for all your players.
Less than 20% of the Israeli population was upset at their government during the height of internal disapproval when the protests were going on. If that's the standard to overthrow a nation, then Mexican immigrants should be in charge of Canada and the US.