I have 2 servers and currently am paying $20 per month on each for a plesk license. Which over a 2 year estimated life of the servers will be close to $1000. A little too high for my liking.

Now sw-soft sell:

30 domain license for $199

or 10x10 domains for $990 ($99 a license if you can get away with only 10 domains on a server, I can).

So my question is, is it unethical / not allowed (by servermatrix) for me to source my own license?

If I am allowed (ie the $20 I pay per month all goes to sw-soft), are the 8 others who need a $99 plesk license (10 domain limited)? (saving me $800 over 2 years!).

You could order servers without a control panel and get your own license. There is nothing in the TOS/AUP which prevents you from doing so.

I believe they are offering it as a value-added service for those who cannot purchase the license off the shelf.