Motorway service stations

Why do the petrol stations on the M1 hike up their prices by several pence per litre just because they know that people don’t want to leave their route to hunt for cheaper. Do they think we don’t notice? Of course we do, but we pay anyway. Why?
Go into the shop at the motorway services and Mars bars are 58pence instead of 42pence? We pay. Why?
We have to stop paying the people who are ripping us off. Of course, 16 pence extra for a mars bar is not going to change our lives, nor is a couple of quid extra to fill up our tank, but it adds up and wouldn’t you rather have it in your pocket than in someone else’s.