That's where the problem lies. Is the solution then that Apple make it impossible to pass on the invite? Or maybe that we speak to our friends and explain what you regard as
It is up to Apple how they want the invite system to work, as long as they explain it clearly.
If it is possible for invited persons to bring uninvited guests, it should be explained in the help documents. We need to know it in advance before we set up an album and invite someone.
The FAQ just says "As the owner of a shared album, you control with whom you share your album."
It is very unexpected that others can invite subscribers to albums they do not manage.
I suspect the reason that Apple allows the invitation to be accepted from a different account is that people may have set up their mail to be forwarded to different mail accounts and may reply from a different mail account. In that case it would be inconvenient to have to accept the invitation from the original account.