Not that this excuses GitHub/Microsoft in any way, this was an obvious outcome and they're morally and legally responsible.
It doesn't matter whether the code is GPL or MIT or BSD. If Copilot reproduces it in your codebase, you're violating the license anyway - almost all FLOSS licenses carry an attribution requirement, which Copilot does not and can not reproduce[0].
The difference between GPL and MIT is whether you have to release your source code, or just add a blurb in README. It's a big one, but it's downstream from the core problem: with Copilot, you won't even know when you're violating some license - much less what to do about it.
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[0] - The whole point of a DNN model is to pre-mix the inputs in training, so that responses to queries are cheap and fast. This comes at the cost of making it impossible to reverse-query the model, so the only way for Copilot to give correct attribution would be to take its output and run a search on the training data, which would kill all the costs savings they won by using a neural network.
And as this famous example[0] shows, the GAN is able to reproduce what is unquestionably copyrighted material from those repos.
[0]: https://twitter.com/mitsuhiko/status/1410886329924194309