Sammy's injury and placement on IR was attributable to pain tolerance (it was confirmed that he couldn't do any more damage to the foot). So he was unable to cope with the pain. The OP may have a point (unlike Rothlisburger who has a torn meniscus, Hoyer with broken arm, etc. more serious injuries).
Dareus is...well, he's Dareus. He can't play because he needs to get himself back into game shape (did you seriously expect him to workout over those 4 weeks?)
I think Shady is taking it easy...he had himself a career game (3 TD's)...he's relaxing the legs for the week.