1Password should by default just always capitalize one word, and add “1” at the end of the memorable password. Since the words are separated by “-“ or “.”, you already hit the “at least one symbol” rule.
One click and I can paste anyway. Nyah nyah nyah nyah nyah nyah.
The purpose, which is to make sure the user knows what they're deleting permanently, is defeated if they can copy the end of the URL string and paste it straight in. Adding a bit of friction there is helpful.
The actual answer to your question is more like "someone thought it was a good idea and now we're stuck with it", though. More browsers should offer a force paste in the context menu, because when said is done, it's my browser, and if I want to do something, I should be able to do it.
While I like the dialogue it’s only a step up over a confirmation dialog (forcing you to switch from clicking to typing). So disabling paste don’t add anything to that. I’d rather they have a trash section so I can undelete or force remove the project.