If the property was listed by the homeowner (which, in Palm Beach County, most "short sales" are) then the listing agent is only required to present an offer to the owner/seller. It's up to the seller to decide which offer(s), if any, to present to their lender(s) for approval.
And while a listing agent typically must present all offers, if he or she has been instructed by his seller (usually in writing) to automatically reject offers under a certain price or with certain terms, he may do so.
I regularly receive instructions to present offers only if they are accompanied by satisfactory lender approval letters, proof of funds, all requested disclosures and addenda properly signed, and properly completed forms. And occasionally my sellers tell me to simply reject anything under a certain amount, requests for seller financing, etc.
(And for Bank Owned, REO properties, sometimes they are even stricter... since they have a lot going on and they can't be reviewing everything that gets faxed to their listing agent)