1 MHz is the speed they run at fresh from the factory. Setting CKDIV8=1 (that is to switch it off) will change this to 8MHz. The internal osscilator can also be calibrated through a CPU register (OSCCAL) and as such can be made highly accurate and suitable also for clock sensitive applications.
If speed is less of an issue (as is the case for a broad range of applications) being able to just drop the external crystal/oscillator simplifies the design and reduces cost.
As for reprogramming existing hardware already operating with a crystal, I can't really see the point. I assume however the original poster had something else in mind.