The reason is simple - battery.

Stability is too but it is not for underclocking like Droid.

Frying has nothing with it - CPU was designed by ARM and basing on common technology licensing practice I guess TI didn't change anything in it's design because ARM did all work - thermal, radio emmision, delay propogation etc. So, TI got ARM design which can run on max freq from 600MHz to 1GHz.

However, the TI production line quality limits an effective max frequency and for stable run use 600MHz. But your mileage varies...

(usual disclaimer: anything here is for education purpose only and is not intended to convince you for overclocking or doing something bad).