GNU disagrees.

> Many people believe that the spirit of the GNU Project is that you should not charge money for distributing copies of software, or that you should charge as little as possible—just enough to cover the cost. This is a misunderstanding.

> Actually, we encourage people who redistribute free software to charge as much as they wish or can. If a license does not permit users to make copies and sell them, it is a nonfree license.

https://www.gnu.org/philosophy/selling.html

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.