The nets aren't a plot to subjugate the dolphins though; for the most part humans would rather not have dolphin bycatch. They're just something that dolphins haven't collectively figured out how to avoid.
That’s like saying humans aren’t intelligent because they can’t avoid car accidents in the rain.
Even if you were super intelligent, i bet you’d get fucked up in a net if you had low visibility in the water, you were traveling at high speed, and you didn’t have opposable thumbs to untangle yourself.
If I was an sonar-equipped aquatic mammal that was also superintelligent, I'd expect generations of losses might have taught my civilization not to swim near boats.
(Unless we're positing that running the risk of being trawled is an essential part of mucking about and having fun, or a scientific experiment, or dolphins having enough imagination for some of them to speculate warnings about boats are a conspiracy to keep them from the best fish :D)
Boats go where your food is. Avoid boats and you'll have to avoid the most plentiful food sources. Either starve or run the risk that you might stumble upon a boat and someone from your group might get tangled.
Even if you were super intelligent, i bet you’d get fucked up in a net if you had low visibility in the water, you were traveling at high speed, and you didn’t have opposable thumbs to untangle yourself.