“I don’t believe a second, compatible implementation of Bitcoin will ever be a good idea. So much of the design depends on all nodes getting exactly identical results in lockstep that a second implementation would be a menace to the network. The MIT license is compatible with all other licenses and commercial uses, so there is no need to rewrite it from a licensing standpoint.” — Satoshi Nakamoto

https://bitcointalk.org/index.php?topic=195.msg1611#msg1611