NUANCE: The spell description for TS says "You point your finger at a target...", perhaps implying that you can't use TS on a target you can't see or pinpoint somehow. However, the most common source of disadvantage on attacks is not being able to see the target. Since the expected damage advantage of TS+Attack over Attack+Attack is small, if your DM rules that you can't TS someone you can't see, then I think the cost of learning this cantrip is not worth the benefit if you intend to use it mainly with free attacks.
Why would a DM do that? I think you are reading too much into those words. A PC can point to a square or an invisible target as long as the PC is not physically or mentally prevented from doing so. It doesn't matter if a PC cannot see an NPC, he can still point at the square where he thinks the NPC is located and that is sufficient for the targeting rules.
It's pretty clear from the rules that one can target an invisible foe, so it's pretty clear that one can point their finger at an invisible foe. I think that the spell would have to explicitly state that it cannot target an invisible foe for it to have that property.