In places in the US there is a 55mph speed limit. This limit is routinely broken. (According to the Wikipedia article, the limit should be higher - perhaps this is referring to one perimeter road?) To illustrate the absurdity of having a law which is not enforced, these students made a film where they obeyed the law.
As the man pointed out. Going at 75mph is accepted, going at 80mph gets you booked for being 25mph over. Surely it's better to have a law that's enforced and understood - so the point they're making is 'move the limit up'.
The reason for the 55mph limit originally was a fuel economy measure following the OPEC crisis in the 1970s. Whilst this is a good motive, it isn't a valid reason when at the same time cars in demand with incredibly low fuel economy (yes, SUV owner - that means you!)
It's interesting that in the film, to avoid people being angry the guy was pretending to use his cellphone - this would just inflame people in the UK!