It is incredibly likely that GrubHub's employees knowingly, freely, and without coercion or duress, entered into contracts specifying their employment was "at-will" and that they could be dismissed at any time and for any reason.
Is it your contention that those contracts should not be enforced?
Libertarians have no qualms with peer pressure to encourage you to act a certain way. Their problem is with the state using their monopoly on violence for that purpose. Boycotting Grubhub, or offering to poach their employees with quick hiring turnaround, are both fair game.
So where were you when HN users had to reach for their fainting couches because people encouraged disassociating from/boycotting Thiel and his business ventures?
