Why do I need to be charged to update my apps?

If you have automatic app updates turned off you must provide itunes login information from time to time. Especially if you have multiple itunes ID's on the computer you sync with. My wife and I have separate itunes accounts but itunes keeps one common library on my computer. we have several apps that we each have "purchased"(free apps like chrome facebook candy crush...etc) on our own separate accounts, but only one instance of each of these in the library. sometimes however the computer gets confused as to who bought what. I dont know when this happens(be it backups, syncs or app updates while connected to the computer) but it does. My wifes phone started requiring my itunes login info to update certain apps but not others, and always the same ones. in order to stop this I had to delete those apps from my wifes phone, then reinstall them through App Store on her iphone. after this all confusion was gone as to who had to sign in to update what app on a single phone.