The problem is that free public WiFi is wildly variable in quality, whereas 4G is usually quite good. Most of the time I just tether even if there's WiFi available since I can use my data plan in most countries and the WiFi is usually disappointing.

Also, many countries now require public hotspots to verify the user by getting them to receive an SMS. In some countries it even has to be a local number, because they have compulsory SIM registration, so requiring a local number lets them tie the Internet usage to an identity.