all up votes cost steem from the total pool

This is exactly the problem. You don't take the money from your pocket and gamble it. You take the money from a pool that belongs to all of us. And the more that is taken to go towards this game, the less there is for other authors. In other words, it is not your money to play with, it is up to all of us to distribute. But if we first put this money through a filter that does its own redistribution then 50% of what goes through there gets stuck as SP.

Maybe this free gambling will attract more people to Steemit and let them invest in SP because there is a minimum to take part.

Free gambling does not exist. What you mean is a free bet, and a free bet is a very hard sell. People don't buy it.

an up vote will cost exactly the same and in both cases it brings satisfactory to the user.

The upvote doesn't cost you anything. In fact, you gain a curation reward back even if you lose the bet. The upvotes accumulated by many people upvoting the post costs everybody else. When you have 100 posts all making $5 rewards and you have one post making $100 payout, each time somebody adds their vote to that $100 post, rewards are taken from the other 100 posts we voted for. We voted less on them, but only because they did not attempt to buy votes.

Please let the market decide if gambling belongs on the platform.

I agree with this. But gamble with your liquid steem or SBD, not the money that we are all responsible for redistributing. That is not the purpose of your vote.