SpaceX

Why did they do this?

Probably because Ariane 6 is a new rocket, and new rockets haven't had the bugs worked out and have a disproportionately high failure rate.

But now, on the eve of restoring European access to space, Eumetsat has effectively stabbed this industry in the back.

That is not too strong of language, either. In its release, Eumetsat described its new Meteosat Third Generation-Sounder 1 satellite as a "unique masterpiece of European technology."

Good grief.

NASA flew the James Webb Space Telescope on Ariane 5 for exactly the same reason -- because it was an extremely-expensive payload, and when they expected to launch the thing, Falcon was immature, and Ariane 5 was mature. I didn't hear people running around saying that the US had "stabbed American rocketry in the back" by launching something on France's baby. Hell, we spent a long time launching stuff on Russian rockets, which I think probably has a lot more potential for controversy.