For PBM's (they control your RX benefits for insurance companies) to also be a pharmacy is a huge conflict of interest. How it does not fall under antitrust laws is a surprise.
That being said, PillPack seem to be very small and was only supplying 1 out 25000 prescription fulfilled by Express Scripts [1] I also think PillPack didn't want to label itself an "online pharmacy" for branding reasons, since "online pharmacies" are usually though of as places where you buy viagras, etc. which is what express script is arguing [1]
[1] https://www.buzzfeed.com/stephaniemlee/pillpack-vs-express-s...
The lack of clarity over exactly why Express Scripts is "blocking" PillPack makes me suspect there's more to this story.
PBMs recently seem to be getting dirtier. I would hope there are enough angry people to effect change after this debacle goes down.
The other side of the story, from the comments:
"Express Scripts repeatedly warned PillPack that it was violating three standards, according to Henry: it labeled itself as a “retail pharmacy” in its contract when it is in fact a “mail-order pharmacy”; it shipped medications to a state where it was not licensed to do so; and operated without a license from URAC (formerly known as the Utilization Review Accreditation Commission), a nonprofit that helps promote health care quality by accrediting various medical care organizations."