If I remember correctly, the limit is simply a function of how the blockchain works. Adding something to the blockchain creates work, that work must be compensated somehow. The SP is that compensation (indirectly), demonstrating that the poster has a vested interest in the long term value of the platform.

I do think there is a possible solution here in getting new users and avoiding mindless spam.... but I haven't given enough thought to it. I know there was a post the other day discussing this. @timcliff maybe?

That said -- people aren't "charged bitcoin" to participate. Steem tokens are on their own blockchain. Purchasing with BTC is one method, but there are other means of exchange, just no direct from USD yet.

Further -- those are allocated to EVERYONE that invests. This isn't a centralized model. The @steemit account has plenty in reserves to pay employees. The only reason a token is worth what it is right now is because the COMMUNITY of investors and participants have deemed it to be worth that much, in the open market. If they deemed it worthless, it would be free to post and eventually the blockchain would die from lack of interest and witnesses.