..if they cannot sell in an atmosphere in which you are a trusted, open, and reliable partner. That is the most powerful position from which to sell.
Your problem here is lazy salesmen who don't want to be bothered dealing with the phoney issues the competition bring up - they just want an easy sell, or they are undertrained and scared salesmen who are afraid they don't know how to counter the phoney arguments....EVERY such issue is a selling point on trust that differentiates your company and your product from the competition. Your company is straight - the competition aren't, because they keep the truth hidden.
Can the sales people really prove that the openness is the reason why they can't win the sales? I doubt it very much - salesmen don't do numbers, don't do proof, it's all hearsay and presenting single anecdotes as universal truth.
And I say this because I was trained by the best, worked with the best, and sold software successfully when everything we sold was 15-20% more expensive that the competition - and we succeeded because we were trusted.
Your Plan B, if you can't get the bosses to back you: close Bugzilla to the public, open it to third-party and developers and (KEY IDEA) to the relevant IT staff at customers. You sales people MUST MUST MUST use the customer IT staff as recommenders - if they aren't, they are NOT doing their job properly.