For example, the reason why my friend could buy Adobe Photoshop for 1/5 of the price was because he is a student and he was forbidden to share trade or sell his license further.
Such low prices wouldn't be avaible without this limitation.
Adobe is within its rights to impose a limitation on the person buying the software at the point of sale - namely requiring that your friend be a student to get it at that price - because Adobe's distribution rights are still in force before the transaction has been concluded. But it has always been the case that once the sale transaction is complete, the publisher's rights to control the sale or resale of that specific licence are exhausted. Your friend could legitimately sell the licence on to a non-student.
The exhaustion doctrine in a nutshell.