I can assure you that 90+% of the time someone gives you a +/- score, its based on an all par 3 formula. They added it that way, for their dissemination, not yours. If its that big on an issue, ask them what their +/- score is in actual strokes.
There is no such thing as "kept track of their score like they are suppose to" during a casual round. There is in fact, no mandate to keep score at all. I've never kept score of my rounds for anybody else's benefit. If they don't like the way that I do it, tough cookies.
Because unless its a tournament, where we're mandated by rule to keep score, its too time consuming for the purpose. When I play solo, I like to keep a brisk pace. I don't have time to write down a score every hole, when I can keep track of an entire round in my head by treating 3's as zeroes and non-3's accordingly. Even when I'm having a "league" round with some local guys I play often with, we use the +/- method, as we've never carried a scorecard with us, and only recently have one of us starting using a mobile scorecard app, which I think was only started out of love of gadgetry as opposed to need.
I doubt that it would be practical in ball golf because most casual ball golfers don't regularly par, or even get single bogeys with anywhere near the frequency that recreational disc golfers do. A standard Par 70-73 ball golf course also offers three different pars on their holes, with maybe just over half of them being Par-4s, whereas the overwhelming majority of disc golf holes really are true Par-3's.