I said the cap could be around 30, not 30, BUT this is a common thing done in many leagues here in the US both at the college and high school levels.I'm not sure if they have it in the professional ranks here. Capping the PD is not uncommon, mainly because of sportsmanship and fairness of play. It would not penalize you as everyone would abide by this rule.
There are no forfeits at the professional or even the collegiate level. I have only been a part of one forfeit at the high school level (which is what i coach) and that was because of gang related shootings that our opponent was tied to and they were afraid to travel to our field. Forfeits at higher ranks do not happen....and if you can find isolated incidents they would not add up to what BB would add up to.
Now, you are correct that the training suits the game, and that's precisely why it needs to be tweaked to make it suit the game better and not penalize teams that are trying to achieve specific training minutes.
What baffles me is that you care more about capping the PD than you care about your weekly training being screwed up. You claim that it penalizes a team for running up a score to cap the PD. Really you could cap the PD at 50 for all I care, just as long as it would prevent the unfair advantage of your example of a 300-14 match.