Wouldn't it just point to their mobile phone instead, which is contracted to a real name and credit card / money?
Not directly. And in many cases, not at all.
A smartphone that has been connected to a WiFi network will default to sending ALL internet traffic over WiFi instead of the cellular network. So, it'll be just like your netbook.
So, if the RIAA/MPAA wanted to file a "John Doe" lawsuit based on torrent tracker records, they'd see that the public IP used on the connection was on the network owned by, say, AT&T. They serve AT&T with a subpoena, and find out that, at the time in question, that IP was assigned to "Mom & Pop Coffee Shop".
At this point, they either name the owner of "Mom & Pop Coffee Shop" directy in their lawsuit and call it good, or they contact them and demand records on who that IP was assigned to.... Records which probably don't exist. In which case, they'll probably say "fuck it" and name the coffee shop in the suit anyway.
The larger chains that contract to third-parties to manage their customer-pointing WiFi (like Starbucks) may actually retain those MAC address records (and email addresses, if their capture page collects them)