If we're all standing on the shoulders of giants (specifically code that other people wrote) then really what Copilot is selling is a ladder to get onto those shoulders faster. I think that's a legitimate aim, as such. However it should be careful about not including unlicensed code and should have a specific 'GPL' option for a model trained with GPL code included.

I suppose it should also generate appropriate copyright notices to satisfy many open licenses. I'd be surprised if copilot could actually link back to the original code like that, though.