Well I believe every organisation needs some from of revenue to keep their business running. Steemit has always taken the option of powering down their stake to sell, which has, in my opinion, negatively impacted the price of steem. So having ads as an alternative means of generating revenue isn't such a bad idea.
'Where does the money come from?'
We know steem has value but why does it have value? This is because people invest their money in it. I think over $50million worth of steem has been sold so far and many don't bother to ask where this morning is coming from. If and when we do we will realise that ads are a necessary evil, then there won't be much fuss about ads. Also, there are other interfaces which you can explore other than steemit, so it is really not a problem. If you don't like seeing ads pop up on your screen occasionally then you can opt for busy, partiko, esteem. Etc you have options.
Steemit is in charge of coding and many important things on the steem blockchain. Which they do for free. Notwithstanding it still come at a cost which they solely bear. So it is only logical that they explore other options as regards generating revenue, and reduce the selling pressure of steem
I hope I have been able to answer your question?