The market is efficient, but only on the long run. People making decisions like this make the market efficient by storing oil when it is cheap (or when it appears to be cheap).
The same thing is happening here:
These ships are trying to move oil from a time when oil is relatively plentiful to a time when it's relatively scarce. It's riskier, but if they succeed, everybody wins: they make money, and they do it by lessening that future scarcity.