I really don't know what the point is, but maybe it is becaus I live in Europe, Germany and see these signs quite often.

It is simply common, that not in all stores or places all bills or coins are exepted. You wouldn't expect to pay a 9 EUR Breakfast at a café with a 500 EUR bill or to pay a 4000,- EUR car with 2,- EUR coins.

As many people posted above most of the gas stations, shops and restaurants have their restrictions and do not accept 500,-, 200,- or even 100,- EUR bills.

An interesting detail for this case is by the way, that the countries where people use bar money / cash the most are Germany, Switzerland and Japan.
It has still something to do with status.
Thats the reason why the 500,- EUR bill not only exists for people to buy their used car or criminals to pay their dope.

Besides all these gas stations, drug stores or cafes at the airports with their self-made cloddy "WE DONT ACCEPT" signs there are many restaurants and bars which discretly accept these bills without batting an eyelid (can u say that in english? I don't know) - I don't mean these really weird expensive bars or hotels, where you cannot expect much change from your 500,- EUR bill after drinking 4 cocktails anyway ;), but some middle class restaurants - steak houses for example.