Vendors accepting bitcoins need to offer a discount on transactions denominated in btc.  They can because the btc will most likely appreaciate in value in the time between earning it and spending it.  Bitcoins is a real hard currency, the hardest, so you do prefer to earn btc rather than funny fiat money but your customers will have the opposite preference, they will rather spend the funny money and save the hard currency.  To encourage them to give you real btc instead of funny money you can and should offer a discount for btc payments.