Regulation.
OK, a stabilised supply for the oscillator, normal. Usually, an osc. has a Buffer attached to it, running off the same stablised supply. Often an emitter follower, to keep frequency stable with variations because of what's going on outside. The osc and buffer is often together in same enclosed box.
Why is the regulator supplying a preamp transistor as well? Doesn't the preamp want as much stage gain as you can get? If the voltage is dropped, the RF gain goes down. And it wastes power. You'd be better off using that power in a Buffer instead, and you'll have more output power.
If you don't have a buffer, and the preamp is acting as a 'buffer', maybe it's over-coupled to the oscillator? If so, no wonder you have to stabilise the preamp supply, because as the supply varies the input capacitance of the preamp varies and messes up the oscillator.