my understanding of this may be to limit the chance of injury during practice, correct?
were there that many injuries caused during practices to make this a pressing requirement when negotiating?
surely there must be ways around it to help develop players that won't get the meaningful snaps on practice?
It's injuries and just wearing guys down over the long season. If preseason is any indication, where starter snaps are limited to just enough to get full contact juices flowing, both players and owners are fine with it since everybody is operating under the same set of rules.
As for ways to develop players, there's a lot of non-contact camp work and then the preseason where players have take what they've learned to the field. Even so, the level of competition other than for a couple of series may be against fellow rookies, other developmental players, or camp bodies hoping to make the PS.
Once the season starts, with the intesity of game preparation, there isn't much developmental coaching going on. Bench players have do for themselves in preparing to play if called upon. The closest bench guys get to developmental work in-season is working on the scout team, where the second string offense and defense attempt to emulate the upcoming opponent in practice sessions agains the first string. Even so, with limited contact practices in-season, I believe a grand total of 12 is the number, there's not a whole lot of opportunity to work on technique.