That easy: make you liable for what people do on your network. Make it hard to register as an official network provider that would forfeit some of the liability.
https://qz.com/694618/why-is-it-impossible-to-find-free-wi-f...
(AFAIR, in Germany there are some changes now, but it's a good example how Internet sharing is made difficult for anyone without a large legal department.)
On an unrelated note, incredibly shady things are currently being done by US corporations in regards to the 5GHz unlicensed spectrum being used for commercial LTE [1]. This is obviously a very slippery slope and would need strong regulation from the FCC. However, the FCC just approved it for live use in February (Pai got into office in January).
[1] https://en.wikipedia.org/wiki/LTE_in_unlicensed_spectrum