Not "for free". Settlement Free Peering is based on a mostly balanced flow of traffic. The instant that ratio moves from 1:1 to 100:1
And THIS is what makes them not common carriers; ISPs can do this. In the Telco world, interconnect fees are required to be symmetric, for example: if the agreement is that charges carrier A $0.05 per call record to terminate onto carrier B's network, then it must charge carrier B $0.05 a call to terminate onto carrier A's network, it's not allowed to charge carrier A $0.05 per call and give carrier B free service into carrier A's network. An interconnect agreement cannot be terminated or repriced to favor specific networks, just based on the ratio of calls in or out.