They do this for free because they're a tier 1, meaning everyone pays to connect to them and they benefit from having more things hosted on their network. It's free because your ISP pays for it.

And they can always tell your ISP if they don't like paying to connect to the tunnelling server they should just implement v6.

Unfortunately, he.net tunnels have terrible reputation, so if you use this for your regular browsing, you're likely to get hit with more captchas than if you're on a cgnat with well behaved neighbors. Some sites will just not work, with no errors, because why waste errors on networks with bad reputation.

It wasn't always like this, but it was for me the last time I tried.

I think I had HE and Surfnet tunnels in the very late '90s already. IPv6 sure is taking some time...

Annoyingly, this will not work over CGNAT as far as I know - the tunnel needs 'both ends' to be accessible directly from each other I think.