Do these publishers advertise the part about "assisting" in the costs? Or, is it just common knowledge? If you're offered a contract with one of these publishers on the condition that you "help" financially, is it bad form to pass on the offer since the LDS market is so small?
I don't think any of them advertise that they offer author assisted publishing. If you're interested, you may need to ask them. In our company, we don't mention it unless the author has indicated that they might be interested in self-publishing.
No, it is not bad form to say no thanks to this type of offer. You don't have to accept any offer you don't like. Here's the thing--everyone knows you're going to make a decision based upon what's best for you. Author assisted publishing is not your best option and should only be considered if all other avenues are closed. Publishers know this. If you want to publish traditionally and you still have options elsewhere, explore them. If you later determine that you've exhausted all traditional methods for publishing your book, you can always go back to the publisher who made the offer and see if they're willing to re-open negotiations.
And don't worry about intra-industry gossip lessening your chances of being accepted by publisher B if you turn down an assisted offer from publisher A. First, we probably wouldn't know that you'd taken a pass on the offer because we don't generally sit around and talk about who turned US down. Second, even if we did know, it wouldn't be a negative. Depending on who made the offer to you, it might even work in your favor. But I wouldn't include the fact that you passed on the offer in your query to other publishers. It's not good form.