Are we talking about the same thing here? I was thinking of https://tunnelbroker.net/
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.
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.