It allows founders to take money off the table without selling out to a company who would have interests other than growing github.
In addition, if founders want to take cash off the table and that is what it takes to make a great investment in github you do it.
A VC wants 10x, I see no better way to allow this than to let a founder take some money off the table and shoot for the moon.
As far as risk is concerned with expansion, as long as the founders and the VC want the same outcome I dont see what the big deal is with taking on more risk, particularly for founders with other peoples money. Even better after theyve taken money off the table!