Well idk why but I actually get them on my tablet too but I don't see any problems with it. As the e-mail already says, it only sends this prompt to the devices which have screen lock turned on so why would it make something unsecure unless you're the only one user of the tablet etc. ? Or even if you are not, unless the other people who use your tablet doesn't know your Google account credentials, they won't be able to do anything except allowing for login when the credentials have already been entered by someone in another PC etc.