I'm not sure the reasons why they are doing it (as is always the case concerning them),

It is elementary, dear Watson. Steemit Inc has employees it has to pay salaries to. It's got an office it rents. Steemit Inc's income from banner ads on Steemit does not seem to cover all those costs. Steem apps have 10,000 active users put together, not all of whom use Steemit. Steemit Inc is not making a huge deal of money from banner ads on Steemit. And that's their only source of income apart from selling their premined stake.