But you can....
Go to 'Where To' > 'Points of Interest' and then, instead of choosing a category select 'Near' at the bottom of the screen. Then select 'My Current Route' (you need an active route for this not to be greyed out) and click OK. You'll now be taken back to the POI category screen where you can search for POIs along your route.
This works well in my experience, so long as the petrol station is still there
You can use the view the POIs on the map to see it's proximity to your route as it also include those a short distance from it. If you have a choice then the one on the route would probably be more suitable.
We used this on the continent this year as I was riding with my m8 on his Ducati that needed fuel pretty regularly. Combined with PMR radio comms it made the experience pretty slick; he's let me know if he was getting low, at next lights I would ask 'Doris' where the next filling station was on our route, would report distance to go back to my m8 and Robert's your Mother's brother. For those filling stations off the route it took us no further than a couple of km at the most, I'm not sure what width of 'corridor' it searches in though.
There are also other options in the 'Search Near' list as well as 'My Current Route', these are; 'Where I am Now' (default), 'A Different City' and 'My Destination'.