What can you see when you change the security?
Once the security was removed, I went to "protection". The link at that point to "restrict editing" was unavailable (wouldn't open).
Then went to "Encrypt" under Protection. The only options were to encrypt with password or certificate, manage security policies or remove security (which had already been done) Under Manage Security Policies, another screen popped up where the only option was to encrypt with password. (which is only for opening the doc) Not sure if it has anything to do with the fact that it's a "form" or not.
For the one created (before the security removal) it allowed all other functions once opened - where anyone could edit the form.
Use File > Properties > Security