Yes, I do it all the time (use other people's WiFi when traveling with my laptop). As for why the Island Cafe didn't trace her IP, the cafe probably aren't tech people and they just offer free WiFi to anyone as a convenience. She easily could have sat in their parking lot and used it and they wouldn't know.
So, can all the IP addresses that went through that Island Cafe be logged? Absolutely. Then the LE would have the task of tracing who used the WiFi during what period of time, and as far as I know that can only be done by getting the MAC address (uniquely identifies every computer and cell phone) assuming again she had a laptop or smartphone.
Technology FYI:
An IP address is 'leased' to your computer/smartphone depending on where you are and can change.
A MAC address is a unique identifier to computers/smartphones - think of it like your home address - it doesn't change.
Would this all be time consuming for LE to trace? No, but search warrants I'm sure were involved and then if TH used a smartphone/device that LE couldn't get possession of (think bat phones), they have to tie that to her somehow.
My thoughts are it would be why they keep coming back to SI - because they somehow placed her near this cafe.
Of course, I have no clue, but it does all make sense from a tech perspective. Thanks to BeanE and Loves2Bmom for the interesting thoughts.