Your first sentence is a fact, the second is an opinion you've arrived at based on the previous fact and is not a fact in itself.
If there was a minimum speed limit on the road then it would be sign posted just as maximum speed limits are. There would be no requirement to maintain the minimum limit, just as there's no requirement to maintain the maximum. For example, if the minimum limit was 20 then you could drive at 30 etc.
The issue on 99% of roads is that there is no lower limit, therefore it becomes open to interpretation. Your interpretation isn't shared by everyone (as proven in this thread) therefore you need to rely on some indicators to help you out when you think someone is driving too slowly.
Have you had to drop into a lower gear?
Are you now driving at the next speed limit (e.g. 40 in a 60)?
If the answer is no to the above, then the car in front probably isn't driving too slowly even if they aren't driving within 1mph of the speed limit. They definitely don't need to drive at the maximum safe speed allowed by the conditions because by definition they are a whisker away from driving at an unsafe speed, and a lot of people aren't comfortable or confident with that. That's why it's unusual to see a minimum speed displayed; there are too many variables involved for that to be a definitive number.