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You are correct winning on free spin is very rare. Plus you still have to do roll overs before you can withdraw if ever you win from free spin. Its just a marketing strategy to get new users.
This marketing strategy wouldn't work if there were no winners who could withdraw their winnings from free rolls. I personally participated in such free rolls, and although, indeed, in some cases wagering requirements were impossible to meet, I did manage to withdraw my winning several times from various sites, and, I'm sure, it wasn't me only.
Every established gambling site runs this or that free roll periodically, and this attracts new users because some of them
can withdraw their winnings. Sites that make it impossible to withdraw such winnings quickly lose users and disappear.
I did try the freespins for bitcoincasino.io ( they are currently running the most ads on bitcointalk) and I ended up winning around $30 and exceeded the maximum play amount and they still wouldn't let me cash out. I was testing out their casino and they end up disabling my account, lol. so sketchy