This is a very good question, which rests entirely on your shoulders as a commissioner.
The SBPS site is strictly set up as a means of sharing information between commissioner and participants, which is entirely legal virtually everywhere in the free world. SBPS does not engage in any direct collection of money to be used for entry fees, and does not facilitate the direct payment of winnings to players who won prizes.
Simply put, SBPS has absolutely nothing to do with your money.
How you choose to use SBPS is entirely up to you. I strongly encourage anyone with concerns to review the applicable gambling laws of your city, state, and country, to determine if running a contest that includes the transfer of money between parties is something in which you wish to participate.