Weren't those tankers operating under false flags? Additionally, the US action in Venezuela led to that stream ceasing. I'm not sure what the deal was with Mexico, I read that the US asked them to stop doing business with Cuba but they didn't seem entirely willing to cooperate.

When a properly flagged Russian tanker came through it was left alone.

My impression is that the situation with Cuba is much more complex than the mass media portrayal of a straightforward blockade. Not that I believe the US is free of guilt here; clearly harm is being caused and the motivations seem suspect at best.