I can see what you are saying...it is not nice to think that your stop getting hit may have been a manipulation.
This is probably the second biggest fear that people have about SB, with the first being "getting thrown off for being too profitable". I hope I have convinced one or two people that this reason is nonsense unless scalping.
But getting back to the price manipulation issue. It does happen, but not as much as people think. In a rolling daily bet, which is by far the most popular, the price tracks pretty well to underlying. I have spent many hours comparing realtime spot fx to SB prices and they track exactly. Only time they may differ is in a MAJOR spike in underlying, when they can delay and sometimes get ahead of the spike. In general though, the prices are really quite good.
They can be a bit worse for futures prices though, becuase people are less able to track the underlying.
BUT, they dont really manipulate prices much anymore because they cant really get away with it. They lose customers every time they do it, so its not worth it. They MAY move their quote by 1 or 2 pips to knock out a load of stops at a roun1d number, but really nothing serious. And no-one on this forum would put their stops at round numbers anyway. Would they?
Overall, the advantages outweigh the costs!!