# Why are many FHSS / DSSS transcievers limited to 63 mW?

While researching a DIY digital video streaming solution for an outdoor quad-rotor project, I've noticed that a lot of the popular digital transceivers seem to be limited to 63mW transmit power:

(sorry, newbs can't post too many links)

My original guess was that it was a legislation issue, but the last module (that operates on 900MHz unlike the other 2.4GHz ones) has the following message in the datasheet:

Selectable 1, 10, 63, 250, 500 or 1000 mW transmit power with a firmware interlock of 63 mW maximum for 500 kb/s operation

That would suggest that something in the underlying physics breaks down above 63 mW for high bitrates. Is that the case, or is it just a combination of coincidence and poorly written datasheets (the third module is rated for 63 mW on its datasheet, but 100 mW on its specs page)?.