I have thought and thought on this topic...
and there are only 2 possibles I can think of why skype still is allowed to work on the n8x0.

1- We dodged the Great Skypenami!!!! hahaahaha... when everyone else was being erased ... and microsoft was getting rid of skype for old platforms... they overlooked our n8x0's. But I find this thought unlikely... since one thing to bet on is that Microsoft is nothing but thorough when it comes to axing anything that could cost them.

So it is more than likely option two is more plausible.

2- Before Skype was bought by microsoft... I believe (and this I think the more likely) that just as nokia - microsoft cannot release proprietary bits to us ...due to binding contracts with other parties such as skype (at that time) and others... I believe that it is quite possible that such binding contracts at that time made Skype bound to honour such contracts likewise...
Sort of a "Skype-Nokia Death Grip" on each other... as the two fell plummeting, flaming.... into the abyss...carrying our n8x0's into a happily functioning Limbo as a result (colourful imagery eh?).

I highly suspect the 2nd possibility ...
since far greater and more widely used and popular devices have been Skypenami'd since.
I do believe some specific bargains were made, babies sacrificed, and demonic pacts done when the n8x0's were devised.