I don't believe it is a FL law or a law that applies to any other states, for that matter but it is a "Code of Ethics" protocol. Believe me, as a Realtor, I'd love to have that edge of knowledge in multiple offer situations. In reality, the reason why offers in a multiple-offer situation are not disclosed is because it would not be fair to all the other buyers who are placing their best offers but the real estate agent tells another buyer's agents "If your buyer wants this property, he/she has to beat $...."
Imagine that you are making an offer on a house listed at $350K. You really love that house and your hopes and your heart are deeply set in getting that house (and that is when real buyers get to make an offer on a property). You place an offer at $335K but you are willing to enter in some negotiations. The seller counters at $345K and you come back $340K because this is really the top amount of money you have. How would you like to find out that, in the process of negotiations, another offer comes in and the listing agent used your offer to get the other buyer to come in at a higher amount...and you lose, not fair and not square?
Today, in Southeast Florida, we are unquestionably in a "seller's market". When buyers are ready to place an offer on a property that is priced right, they shouldn't dicker or lowball their offers anymore. I monitor sales activities in our MLS every day and it is evident that sold prices vs asking prices are only about 2 to 4% from the asking price, anymore. The sellers are back in the drivers seat. And I hear all the time from procrastinating buyers, "Sh.., I should have offered a bit more."