/etc/shells contains a list of binaries that the system considers (unrestricted) shells. That means that any user that has configured one of those binaries as their shell is assumed to have full access to the system (meaning they can execute any command, provided they have the appropriate permission).
The most direct result is that they can use
chsh to change their configured shell.
If a user has a shell configured that isn't in this list, then the system assumes that he's somehow restricted. In the case of
chsh it means that the user cannot change that value.
Other programs might query that list and apply similar restrictions.
So by putting
/etc/shells you effectively say "any user that has
nologin as its shell is considered a full, unrestricted user". That's almost certainly the exact opposite of what
nologin was meant to say.