Incorrect, steam allows for lower prices and give aways if steam keys you request... As long as it's not the base price. That's how humble bundle works. That's how every dev give away works.
The "devs" in this case want to sell access to the steam version of their game for lower than the steam price, on a permanent basis. Which is against steams rules, because steam provides a service that needs to be paid for, one that is worth far more than the 30% cut.
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.