It's not that they don't allow the use, it's that they don't have a convenient 1Password icon next to the password field. I've noticed some apps have that. Not sure if it requires some specific integration or some open protocol.
The password manager integration is a public standard. More annoyingly, though, apps can also find out if you've pasted into a field and then immediately clear it (some rinkydink banking apps do this.) So it's a 2-pronged problem.
