You are not totally wrong if you say that exchanges and market makers decides when SL will be hit. It has happened so many times in the market and sometimes it will look as if those guys are just watching you and immediately the SL is hit, the market will reverse immediately. There are alot of manipulations in the market and I think trading has alot of manipulations compared to gambling. The only known manipulation in gambling is match fixing which is rare in major leagues.
Well, from time to time, there is news about fake matches in one sport or another. And this is a real problem for those who are betting.
But in general, yes, there is much more manipulation in trading than in gambling, and the big players really know where there is a large concentration of users' orders.