Create a libfoo1 package, have it include the necessary library files (and perhaps a -devel subpackage if you wish for things to compile against it) and update the base libfoo package to the new version.
Ok. So you can/do create a libfoo$major subpackage at times when it's necessary. It's just not required/recommended to always package libs that way. Fair enough.
I guess it just seems to me that it would be more consistent/less surprising to have libraries always follow this scheme rather than to do so only intermittently. But that's just my 2 cents.