Flight planning from my perspective is choosing refuelling stops that have a swipe bowser. There can't be any justification to charge multiple fees when the aircraft are lined up waiting.
Some posters allude to the fact that it's the call-out fee that feeds the refuellers family, which really is crap. They would be on a wage paid from the fuel sale, and pilots have always been a captive audience when it comes to price.
The call out fee must be a lucrative income, considering the vocal resistance when there is any mention of installing swipe bowsers.