Of course you're right that users have a choice how to handle their money on Steem, but I don't see that as being why Steem isn't a ponzi. The only ponzi scheme I know of where people aren't given a choice whether to 'invest' or not, but are forced to at gunpoint, is the United States Social Security program. Other countries may have similar scams, but I'm not familiar with them.
Steem isn't a ponzi for a number of reasons, but perhaps the most clear is that there's no requirement to invest externally valuable resources. One can build stake in Steem simply by spotting good posts first. There's no need to buy in. Moreover, I don't think anyone is really pushing people to buy in with external funds. Sure, we love it when people do, but no one (at least, no one at Steemit Inc) is going around claiming it's an awesome investment opportunity.
It's impossible to classify something as a ponzi when no one is actually asking for your money. :P
Respect the Hustle - STEEMIT is here!!
Full $TEEM AHEAD!
@streetstyle
Thank you very much for your post and comments on this subject and I hope users will have a better understanding on it.