An IP address is light-years from a real-life identity. Without a warrant, there's not a chance of figuring out even the account holders name-- which could be different from the person who actually performed the transaction. It could be a neighbor, a friend, a roommate, anyone else who ever had the wifi password, the possibilities are endless.

If anonymity is a concern getting a court order doesn't sound like a big deal. Even if not, you vastly overestimate the anonymity an IP provides, and if anonymity is a concern legal deniability isn't a big win either. You also ignore that you could potentially get dozens or hundreds of IP addresses. Anonymity, like security, is hard, and has layers.