[dsr_]

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.

[bombcar]

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