Why must i apply the business percentage formula to the rental car fees for rideshare driving when i pay the same amount regardless of how many miles i drive?

[AnnetteB6]

You must prorate your vehicle expenses between business use and personal use (miles when you are not driving for business) because you cannot claim expenses for personal use as part of your business return.  Therefore, you can only claim a portion of your rental car fees based on the breakdown of business miles and personal miles.  

It is the same principle that says you cannot claim all the food you buy in a week as a business expense, only the food purchased for snacks for your customers.  

@riace85