The independent clause
It would have been better...
refers to a point in time in the past when a condition was not true.
To mark the fact that this condition was contrary to the reality, the tense is shifted towards the past relative to the point-in-time in question. A past time which is prior to a point in time also in the past is marked by the past perfect, had banned.
It would have been better if the Donner Party had brought more rations along back in 1846.
If you wish to refer to a condition that is contrary to fact now, at this moment, the independent clause becomes
It would be better...
and the tense of the condition shifts backwards in time relative to the present, and thus the simple past is used:
It would be better if cigarette manufacturers stopped marketing to children in the Pacific Rim.