They will be happy to cease use of the code.

The damages probably amount to $0. "Don't use this code, no seriously don't" isn't exactly an advertisement of the value it provide.

They may already have, looks like that archived repository has been replaced with: https://github.com/heroku/heroku-buildpack-java

which doesn't even use go.

And they'd like you to even move away from that replacement to an ever newer replacement: https://github.com/heroku/buildpacks-jvm

There could be many reasons for having an injunction against using code.

Welcome to America in one sentence.

I really don't understand this way of thinking

I'm no lawyer, but don't you usually have to show damages?

Damages would be loss of revenue. He was deprived of the ability to negotiate terms of use for payment.

This is why the GPL is more developer friendly than other licenses. If a business doesn't like the GPL you can offer to relicense your code, ... for money. And money is the restitution you seek when the license is violated.

I'm curious how damages would be calculated in a case like this. Obviously you can't get away with saying "I would have charged them $10 billion for this code, so that's what they should owe me." But on the flipside, should they be able to say they wouldn't have been willing to pay anything for it, therefore they owe zero?

I am curious as well. Since it's not a typical work for hire relationship, it could be a (modest) percentage of profits, or a business contracting fee of ~$500k or so.

Statutory damages can apply, but in the US statutory damages for copyright infringement require registering the copyright prior to the infringement.