Then they should switch their network to a more complex authentication scheme. Multiple SSIDs, run 802.1x authentication (maybe with certs), MAC filtering maybe (bypassable sure, but will foil most casual users), run their own simple portal and shunt different classes onto different VLANs with different levels of isolation and traffic shaping, etc.
Ultimately if there is one password which then gets any device on, well that's that. Sharing has always been pretty trivial. If the owner of a network wants more fine grained and powerful security, all the tools exist to do that already, but they'll have to use them.
The idea that more sophisticated authentication is possible to prevent password sharing would be a surprise to most laypeople I know.
Also home internet is increasingly coming with smart routers that will alert you to new devices on the network. Tech savvy people are the only ones who BYOD.
Scenario: people come to my home, I give the password to a friend of mine that never visited me before or changed phone and asks me for the password. I don't know the other people and they don't ask for the password, but they get on the network anyway? Principle of most surprise and least security.