The main reason for satellites to be in the Van Allen belts is because the synchronous orbits are located there.
The GPS satellites orbit at around 20,200 km altitude above MSL because the orbital period at this altitude is 12 hours and the satellite returns to the same place on earth twice a day.
At the other end is the geosynchronous orbit at 35,786 km above MSL, where the satellites have an orbital period of 24 hours, thus returning to the same place once a day.
An important thing about the synchronous satellites is that they trace the same ground track every day (since their rotation matches with that of the earth's) and this makes tracking easy. For example, the geostationary satellites make satellite broadcasting to individual households possible.
This also means that they can used to cover the same areas everyday.
Though it is possible to locate the satellites in an active geosynchronous orbit, it requires continuous expenditure of some form of energy. Since lifting payload to space cost astronomical sums of money, it is better to safeguard the satellites against any radiation rather than getting more fuel there.