Here’s from the Operations Manual, “according to an NFL spokesman”:
The National Anthem must be played prior to every NFL game, and all players must be on the sideline for the National Anthem. During the National Anthem, players on the field and bench area should stand at attention, face the flag, hold helmets in their left hand, and refrain from talking. ... Failure to be on the field by the start of the National Anthem may result in discipline, such as fines, suspensions, and/or the forfeiture of draft choice(s) for violations of the above, including first offenses.
http://time.com/4955704/nfl-league-rulebook-a62-63-national-anthem-rule/
I see now that it’s fairly squishy. They “must” be on the field, and “should” stand, but failure “may”, not “shall”, result in fines &c, and only for not being on the field.
Which makes the Steelers actions punishable. without any squishy words.