Out of curiosity how does that work (never been in a situation where I have to worry about minimal fuel purchases) - does the pump simply round up the price to the minimum when done, or did you actually have to dispense extra fuel, either into a jerrycan or donating it to someone else's car otherwise the pump would not actually finalize the transaction?
A google seems to show that it is because the pumps are not certified to be accurate below 2 litres (or 5 litres for higher flow rate pumps like the HGV diesel pumps) so anything below this is at your own risk.
Another way of thinking about it is you have no right to complain if you think you've been short fueled if you don't dispense the minimum stated.
Under that amount, you are likely paying much more per litre.