The difference in the minicamp from other workouts taking place this spring is the "mandatory" aspect, meaning player are subject to fines for missing sessions.
While most players participate in "voluntary" portions of OTAs, those that skipped -- for personal reasons or contractual concerns -- are likely to show up to avoided those fines.
A veteran like Sproles missing some voluntary workouts is not a big deal.
Cox's situation is quite different. The defensive lineman wants a new contract -- and deserves to get paid among the top defensive linemen. Cox apparently won't hold of of mandatory minicamp as leverage to force a new deal.