The fact is that unless you have an employment contract or union agreement that states otherwise, your employer is not legally obligated to pay for your mileage. Such payments are discretionary. Accordingly, to the extent that it does pay them, it has a great deal of discretion over when and if they are made. Basically, in an "at will" employment relationship, a company can set the conditons of the workplace much as it sees fit (absent some form of legally actionable discrimination).