There isn't a hard and fast rule about this but, indeed, it seems like that's the way things are -- 10-power scopes go on sniper rifles.
If you'll look at the history of optics, though, you'll see that many different fixed- and variable-power scopes have been used. However, if you've done much long-range shooting, you know about the effects of mirage and what it can do to your vision with a scope set on its highest magnification. Typically, you'll do better at making the shot if you dial the scope down to a lower setting (preferably, you have a scope that does not allow the point of impact to shift as the power settings are changed). Further, because variable-power scopes have more internal parts (and therefore more potential for problems in the field -- which is where you do NOT want problems), it is often to a sniper's advantage to have a fixed-power scope that offers sufficient magnification to allow the shooter to engage his target. Everything is a trade-off, of course, but 10-power scopes seem to be "the" fixed-power choice among snipers, whether military or police.
Another point to consider is this: mil-dot scopes. The dots are calibrated to give a sniper the range to his target, at a specific power setting on the scope. You cannot expect to range accurately to the target -- with only one dot size -- with the scope set at 3-power and then also expect to be able to get an accurate reading -- with the same scope and the same size of dots -- at 10-power. It just doesn't work that way. So, between the size of the dots in a mil-dot scope and the need to have sufficient magnification to engage a target at long distances, a 10-power scope (usually, also, a mil-dot version) is often the scope of choice among snipers today.
As for the rifle you mentioned, I have no first-hand knowledge of the weapon. However, with proper handloading techniques, most rifles will shoot at least one particular load much more accurately than others.