Assuming a NO attitude right from the start is probably not the way to go. Telling them to "Pound Sand" or just plain NO is not in your best interest. First, you should let the B investors know about this - ASAP. Second, you should start by emailing the GP at firm A and asking him to clarify why they think "you should pay" their legal fees.
With this information, you should consult your legal counsel on how is best to proceed. Most likely the A VC firm feels like you didn't negotiate in good faith and you made them spend money with no intention to accept their offer. You should in this case talk to the GP at that firm and show him that you wanted to close a deal with them and you even made them aware of another offer and asked them to reconsider the terms.
Most of this issues are easily solved if you discuss things with the other party. Assuming a NO attitude from the start, just brings everyones worst character and might land you in a legal dispute.