Yes, it's called contract law. If you don't pay HE, you don't get a connection to them.
I forgot one detail: your ISP could pay a different tier-1 ISP, as they all interconnect. Nonetheless, your ISP pays top rates for that traffic - tier-1 routes are usually last-resort routes.
Obviously if the ISP is buying transit from HE, they'd have to pay for it, but it'd be surprising if HE was strongarming their customers by adding a clause that's like "oh also, if any of your customers use our ipv6 tunnel, we'll charge you $x/user/month" or whatever.
Buying transit from tier 1 is the only way to be connected to tier 1 which all ISPs are required to do to guarantee they can access the whole internet.
HE in particular likes to set up incentives to draw more traffic through themselves. They do free peering to small networks at many IXPs (this doesn't give you full internet access like transit does) and tunnelbroker is also one of these things.
It really depends on the peering contract. Most are not for transit, but rather just destinations, and generally the side that sends more pays, so that means more traffic to HE if tunnels are in use.
I forgot one detail: your ISP could pay a different tier-1 ISP, as they all interconnect. Nonetheless, your ISP pays top rates for that traffic - tier-1 routes are usually last-resort routes.