UIGEA, [1] the legislation that was used to take down the online poker sites, contained an exemption for fantasy sports. However, that exemption is only valid if the business is operating in compliance with individual state laws. There is an increasing sentiment that DFS is sports gambling under a number of state laws, and many people are just waiting to find out which state will prosecute first. At the time the UIGEA exemption was created, legislators hadn't even contemplated the concept of DFS, and the very legislators that wrote the exemption have expressed concern over it.
Since investors with this kind of money have done their due diligence, my guess is that they just want to ride the revenue train until the inevitable state-by-state crackdown occurs. It's no coincidence that this investment is occurring just before football season, which is their biggest revenue driver by orders of magnitude.
[1] https://en.m.wikipedia.org/wiki/Unlawful_Internet_Gambling_E...