I think this might have something to do with the app being installed as a user app vs system app. System apps require higher privileges to modify I think. Plus, and I think this is important, if a "tech-savvy thief" get's physical access to your unprotected phone, wallet would be one of your many problems.
I don't know about you but my gmail is my life. If such a thief can access it, it would grand him/her enough access to reset my bank password, car insurance info, my valuable digital data, access to my email list (phishing attacks on people who trust emails that come from me), reset my facebook password, twitter, etc....
also there are many other equally important applications that you might install on your phone that rely on only the phone's security to protect your voluble data. I am sure you can think of more than one.
You also have no idea what a true hacker can do to a piece of technology, such as your phone, if they have unrestricted access to it. for example A hacker might be able to root your phone, dig into the system files for some saved unhashed password, deleted (but not overwritten) data, etc. I saw an article a while back about a few hackers in some university that can pull a password for an encrypted harddrive by reading it off the memory after they freeze it with liquid nitrogen (but I doubt you have anything on your worth that much trouble
so in short, PUT A PASSCODE ON YOUR PHONE, if you really want to protect the data that is contained within it. The only other choice you have is to keep sensitive data out of your phone.