Here's the scoop. Per the Herald's Jeff Howe, Patriots cornerback Malcolm Butler's flight back to New England was delayed due to weather and he was late to the first organized team activity (OTA) on May 26th. As a sort of punishment for being late, Bill Belichick withheld Butler from participating in field workouts until, according to the Globe's Shalise Manza Young, he was allowed back on the field this week.
In total, Butler showed up late for one practice, and then Belichick forced him to stay inside and study at the stadium for the next five meetings, per ProJo's Mark Daniels. Belichick typically sends home players who show up late, so keeping Butler in the building was intentional.
The Collective Bargaining Agreement contains specific language that the Patriots are under review for potentially violating. This language is used throughout Article 21 (h/t to Miguel from PatsCap.com) and is as follows, emphasis added:
No club official may indicate to a player that such individual workouts are not voluntary, or that a player's failure to participate in such workouts will result in the player's failure to make the Club (or that a player's failure to participate in a workout program or classroom instruction will result in the player's failure to make the Club or result in any other adverse consequences affecting his working conditions).
Effectively, the Patriots are under review for whether or not preventing Butler from participating on the field, but allowing him to continue to use the facilities, to actively participate in meetings, and to use the time not on the field on film review, constitutes a violation. Specifically, the review will likely cover whether or not this punishment implies the workouts aren't voluntary, and also whether or not they forced Butler into an more difficult scenario to make the club.
By not allowing Butler on the field, it's an easy argument to say that the coaches put Butler in a disadvantaged situation when compared to the other defensive backs trying to make the roster. However, the fact that the Patriots allowed Butler to use the facilities will be used to defend New England in the investigation.
Coaches should have the prerogative on the utility of players at camp, and whether a player receives first team reps, is asked to join the special teams unit on the field, or is forced to take a lap as penalty for a mistake all impacts whether or not a player will make the team.
Butler was still participating in all team events inside the building; if a coach forced a player to work with the strength and conditioning coach while the other players were on the practice field, would that generate any notice? Probably not, and that will also be used to defend the Patriots.
Butler was participating, just in a different capacity. It will be up to the investigation to determine whether the rule will be applied to determine whether what Butler was doing counts as "participation" (it should), or whether the Butler's separation from the rest of the team and its connection to his lateness doesn't align with the voluntary nature of the workouts (it very well could).
If the Patriots are found guilty, guess who gets to determine the Patriots punishment? Old friend Roger Goodell. The first violation counts for $100,000, per Article 21.8(d)(ii), but as DeflateGate shows, the definition of "first violation" is very fluid and has no impact on the severity of the Commissioner's penalty. A violation would also result in the cancellation of the subsequent week's scheduled OTAs, and if the team is found guilty of two violations, they will also lose their 4th round draft pick.
Hopefully this never reaches that level. This is ridiculous enough as it is.