You could respectfully think that Bergmeister kept himself within the rules because he moved toward one side and one side only. That's not blocking in the
Sporting Code I know.
Perhaps Magnussen knew he didn't have the time to tap the brakes, move towards the right and overtake him before the finish line, so
he kept going towards the left side. Unfortunately that was the same side Bergmeister was allowed to move.
You could bet that in any other lap, Bergmeister could have given him room to pass, maybe out of courtesy (not because he was breaking any rule), and more in fear of damaging the left side of his car, but, c'mon, it was the last lap!
What do you expect in the last lap? If you drive defensively, you know that
anything can happen in any lap, but
something will happen in the last one (unless you drive cars with
real power: then you have more trouble with the car than problems with your opponents, and last laps are, usually, a solitary bussiness).
I think last lap "enthusiasm" is much worse in NASCAR and nobody's banned for a day, much less for life. Karters also have a kind of kamikaze style in the last lap, if you ask me. At least my friends seem a bunch of crazy falcons chasing a dove, fighting more among themselves than with the guy in front.
Stock and ALMS are slower than F1, so, in my book, it's not a criminal move, it's merely temerary. That's a temerary move by
both drivers: you could think that Magnussen knew he had no room.
It's not like they started the last straight side by side, you can move sideways once, and I think that if you don't know you can, you will lose every kart race with a close final. Maybe if you watch enough "smaller series", like BTCC, DTM, V8 Supercar or ALMS you could learn more about the fine line between legal and illegal moves: some seem frankly rude at first sight.
The spirit of a gentleman includes certain leniency when competition is involved, but also includes the strange bravery that leads you to receive hard "legal" hits with a smile and with the aim of learning. There are some situations you shouldn't poke your nose in (or if you do, you do it at your own peril, either for the team or out of desperation).