Anyone who has played football at the collegiate level, and even by-and-large at the high school level, knows that voluntary workouts are...well, you are expected to attend. If you have a great excuse, fine. I don't recall seeing that Richardson offered any such excuse.
The CBA says you don't have to be there. Now, in some cases, like HS and college, the response to you choosing not to participate in voluntary workouts is the coach volunteering to choose to keep you off the field. It is a lot tougher to do when you're talking about one of your best players. You can't, and won't, bench him for blowing off these workouts.
But in doing so, you're creating a hierarchy. Better players can get away with more than lesser players can. It's not good for team chemistry, and it certainly sets Richardson as one of the guys who should not be used as a leader by example to younger DL like Leonard Williams.
Mo's situation is far more understandable than Richardson's absence.