Because they are borrowing (licensing) our proprietary technology for their patent. You can't do that for free. The Licensor (ENZC) must be compensated and agree to the particular use of its proprietary information/technology.
They are all Gilead employees- that's how it works- this is a Gilead application for patent, not an ENZC application and does not affect the validity of OUR/ENZC's already issued patents