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From GPLv3 Wiki

Let's assume you allow giving out the source by internet, when the binary comes via hardware-download - I mean: Post order.

Now the recipient sits in China and the chinese government "accidently" blocked the site with the source.

This would mean, the chinese people had no way at getting these sources, even though they could redistribute the binaries, and this would be supported by the GPL.

So I'd rather keep it clear, that a recipient must be able to get the source in the same way he/she got the binary (plus paying a fee equal to the cost of conveying the source).