Pomee's omission from the roster is due to his arrest in early February. He has been removed from all team activities in the meantime. It is unclear why Heyward, Thomas and Wilson have departed.
None of the four received significant playing time in their careers, but losing them will definitely hurt USC's depth. The Trojans are already limited to 75 scholarship players due to NCAA sanctions, and coach Lane Kiffin has exacerbated that issue by signing under the yearly limit in his last two recruiting classes.
All four of these players were from the 30-man recruiting class of 2011, which was supposed to bolster USC's depth heading into sanctions. However, that class is now missing eight players, and several others have either been injured or haven't lived up to expectations.
The Trojans open spring practice on March 5, but with such a thinned-out roster, one wonders how much Kiffin will actually be able to accomplish.