It likely has something to do with software sales amd the resale scene, in that maybe potentially someone could just use eshop cards to buy all their games, unlink an account from the console(assuming that is possible since i believe you can have multiple accounts?), and then sell off the second switch with a bunch of games on it.
No doubt it needs to be limited in some way, otherwise people will abuse the hell out of sharing. I'm sure I've read there are limits to how many sub-accounts you can have active on PSN and Xbox? Seems like a limit of 3 or 4 would be reasonable since lots of people have at least a couple different gamers in a household (last gen I used 2 systems simultaneously all the time, that was it - but it was a nice perk for my digital library to be shareable on 2 consoles for co-up and online and such). If it's a sub-account maybe they can just require the license renews every year or else the games are no longer valid without login?
Apparently Steam is pretty crappy now about "sharing" games as well even within a household, basically Nintendo level restriction where you can only access the entire library 1 person at a time, even if the two people want to play entirely different games! Dumb.