Why shouldn't steem/sbd get a piece of that action and a good piece too?

It is, as long as the steem price doesnt go down, people are entering it because a large amount is issued and sold daily.

On the other hand I don't really get the whole pegged one SBD to one dollar in a totally free market.

The free market and openly traded things are why pegging something is so difficult. The current system we have doesnt do a fantastic job of addressing the system quickly, but pegged currencies in the real world you would basically see governments print more until it reflected the peg. Although for the most part pegs and free markets dont tend to mesh well.

What do you mean by "invest in the ecosystem" and how would that be done?

There are many approaches that could be done. We could pay more to people who 100% power up their posts fully. Stop paying curation rewards to people powering down. Starting a fund where we take 5% of all inflation and fund projects built off the steem system. I think probably the easiest would be the lower the inflation rate of steem. Theres a million things we could do. Im sure people have better ideas than me though.