Why can't I add a user to an account without specifying a permission set if that user is already in a group that's in the account with a permission set?

To repro:

• Start a new account
• Agree to the IAM and organisation stuff.
• Make two standard permission sets: SystemAdministrator and ViewOnlyAccess.
• Make a group and a user, the user being in the group.
• Add the group to the account with the SystemAdministrator permission set.
• Try to add the user.
• Notice that you must choose a permission set in order to add the user. I think it should be OK to add the user with no permission set cos they inherit the SystemAdministrator permission from the group anyway.
• Choose the ReadOnlyUser set
• Log in as this user and notice that both SystemAdministrator and ReadOnlyUser are offered, thus showing that indeed the SystemAdministrator permission set was inherited from the group. Choose SystemAdministrator and launch an instance to further confirm the same.

Seems like a bug to me.