The problem wasn't so much that you could share access to your network with your friends - it was that if you gave your WiFi password to someone (which what the majority of people do when they visit someone elses house) then you had to make sure that they didn't share access to your network with their friends.
The problem is that Microsoft cannot differentiate between someone who has the WiFi password because they own the connection and someone who has the WiFi password because they were told it. Microsoft made the assumption that if you have the password, then you have the right to offer that connection to others - but this is not what happens in the "real world".
Because of this incorrect assumption, the onus was suddenly placed on the owner of the WiFi (who does decide to provide their password) to police the entry of it into Windows 10 devices to ensure that a bunch of random people that they have never met aren't suddenly allowed to use their network.
That was why it was an issue.