Why would the owner of this contract get 50% of deposits?

This is clearly scandalous and that's why this thing doesn't make any sense.
Create the same contract with no fees to the owner, and reward just all the previous participants, that's all.
The owner doesn't deserve anything here, this is just in contradiction with this principle !

This is an experiment and you are free to participate in it or not.
This is not a zero-sum game, it is first-and-foremost a registry of advertisements for money-making games. Use of this contract should not be expected to make a direct profit for most participants. 50% of the low cost of entry is shared with the other participants as a friendly bonus and small extra incentive to join.
If you would prefer to write some original software to your own specification, thoroughly test it, get it verified everywhere, create an original web interface for it, and then give that away for free then that's entirely your choice and I fully support your efforts and wish you luck.
If, however, you'd prefer to save yourself a lot of hours and effort to advertise your money-making games, I invite you to take advantage of this contract.
The choice is yours. Have a wonderful day.