If that "get better faster" mechanic you're talking about is a reasonable amount of time, then sure, it wouldn't be pay-to-win. But when you have to grind 105 digging hours to get a BAR, then it becomes an issue. And if everyone had BARs and PBPs the game would be pretty balanced, considering everyone would be on an absolutely even playing field. Even making the comparison that if everyone would have a weapon would make a game unplayable implies that people believe the gun is overpowered to the point where facing against it isn't fun. And since players want to have fun and be on an even playing field with the rest of BAR/PBP users, they're essentially forced to either grind hours upon hours of digging or pay.