Because that terminal is "input terminal", and, both the individual sources will send current (although very small) "into" the op amp due to their voltages . Therefore, vin has been added.
this is no joke and ment in a positive way - he is from indian ancestry and can pronounce the English language so that one can actually understand him. That is why saurabh himanshu asks if he is indian. Indian people are very creative, but it is normally extremely hard to understand their English, because they are very bad when it comes to prenounciation.
