They make money off of X% of N travelers per day stopping to buy a coffee, cigarettes etc. If you require a substantial amount of space to be taken by travelers who sit there charging for 30 minutes, you're reducing peak travelers (eg during the morning and evening commutes) and significantly reducing total traffic. Claiming that they're all going to come in for a slurpee and this will happen to work out neutral or positive for the gas station is pure handwaving.
