>> Why should someone who can use both hands equally well be said to have two right hands?

If we take word etymologies literally, then the "two right hands" definition would seem absurd. If, however, we view the "dextrous" in its more figurative sense of skillfulness with one's hands, then the word histroy works quite well.

Left handers are not strictly "sinister," nor are right-handers always dexterous. Frankly, I'm glad language is so colorful in its imagery and metaphor.