Fascinating, from skimming that, it does indeed appear that it would be within the GNU philosophy to distribute source code solely in exchange for payment. Doesn't cover a case where the source code is _already_ distributed though, then it's free to run.

And even if the source code was only distributed to paying customers, that'd likely be a temporary situation. A relevant quote:

"With free software, users don't have to pay the distribution fee in order to use the software. They can copy the program from a friend who has a copy, or with the help of a friend who has network access."

I do read the GPLv3 such that if someone _does_ buy the code in any fashion, you must provide the source code to them for free. Relevant excerpt from section 6:

"[...] give anyone who possesses the object code either (1) a copy of the Corresponding Source for all the software in the product that is covered by this License, on a durable physical medium customarily used for software interchange, for a price no more than your reasonable cost of physically performing this conveying of source, or (2) access to copy the Corresponding Source from a network server at no charge."

But yeah, no obligation to provide the source code for free to non-customers, fair point. Just no ability to stop customers from sharing it with non-customers. Does make sense.