What example do you have of a company purposely slowing down a peering link? If you're thinking Netflix, that isn't a very good example.
Would you rather have every packet coming into the ISP's network through transit links only?
There is no double dipping. There are two ways to get connectivity for Internet. One way is transit. Transit can be expensive especially for companies like Netflix. Transit is pretty simple, you pay for x-bandwidth or port speed. The other option is to peer. For peering, one company/network pays the destination network to deliver the packet. This is less costly than transit, but the packet's destination must be within the peered network. If the networks have nearly equal traffic between them, then they call it a wash and don't charge the other party. Nothing new here. This is similar to how long distance companies work. When a long distance company couldn't connect the call directly (they didn't own the last mile), they would pay the local exchange to complete the connection. Last mile networks are expensive. Delivering a call or a packet for that last mile is valuable.