It doesn't make sense to "concatenate" one absolute path to another. / has a simple explanation: if you start at /foo, and then do cd bar, you'll end up in /foo/bar. But if you start at /foo, and then do cd /bar, you'll end up in /bar.

You mean, some of your users write '/some/path' when they mean 'some/path'? Then the users should be educated about the difference. These are not the same, just like '../some/path' is not the same as them, and '~some/path' is again something very different.