They are there because the cafe likely offers free wifi. The cafe wouldn't have to jam wifi there to expel those types, they could just charge or turn off their wifi.
They could also be there even though the cafe doesn't offer free Wi-Fi. Maybe they are getting it from next door. Or maybe they are getting it from a wide-spread commercial Wi-Fi service that they pay for, which has hotspots all around town. Maybe they are on a mobile network.
In other words maybe they are just there for the comfortable seating, and the relaxing smell of vanilla and cinammon, while you, a paying customer, have to have your order "to go" for lack of seating.
If the shop jammed those parasites the heck out of there, I'd consider that an attractive feature of the shop.
