It's good will, technical education and marketing of complementary services. HE sells Internet connectivity and datacenter colocation, B2B. Increasing the number of technically sophisticated people who like and appreciate HE produces more opportunities for sales.

Also, it doesn't cost them much in exchange for lots of good will.

It also gets them some IPv6 traffic that they can use when negotiating transit, which is good for them.

I believe hurricane electric's IPv6 tunnel program was a strong factor in them becoming the de facto connectivity hub for IPv6, helping them establish peering relationships with other large carriers for v6 at least and sometimes they get v4 peering as well, but at least they get a relationship. You're not really on the v6 internet unless you have connectivity with he.net, so most carriers will connect with them for that.

That connectivity makes it easier to sell their transit services, etc. If you're a BGP speaking network and connect with them for v6 transit because it's good, you may as well try their v4 transit too, because you already established a relationship, right?

They're focusing on the enterprise segment (in general, the company running this is a known tier-1 corporation), which will pay $$$ for a reliable and uncongested connections.