The US were not thrilled about it when it was being constructed, obviously, but this was normal tensions towards Russia, prescient in the end but here we are.
There were several countries arguably interested in getting rid of that pipeline (Ukraine, Poland, the US), but Ukraine wanted it the most, had easy access, and there's no need to overcomplicate internet theories.
Yes, it's an operation that requires coordination and planning, which is why it's reasonable to assume it was carried out by an intelligence agency and not a lone fisherman with a grudge. But once you're in the realm of intelligence activities, this isn't exactly the "let's blow up their pagers" level of complexity.
Bringing that bridge down is also much harder than blowing up the pipeline, because the bridge is covered by a lot of defenses, and naval drones will always have limited payload (if they want to be fast enough to evade defenses). Dudes performing a dive in the middle of the sea far from the battlefield are much less vulnerable.