I think is fair also , If you don't want to take the time to drive to them when they have an incall location ,, then you should be willing to pay for them to come to you .. unless all they do is out calls ,, then they don't have the cost of an in call location or pay agency fees ..
Any other profession charge for traveling time , plumbers , electricians , carpenters ,, Your Dr if you can get them t do a house call ..
Weather they drive them self or a friend drives them it still is an expense , You usually are told up front ,, so you have the chose of hiring them or not ..