Why?  I've read it several dozen (if not more a hundred) times over the last quarter century.

Rule 408 says, "Evidence of the following is not admissible — on behalf of any party — either to prove or disprove the validity or amount of a disputed claim or to impeach by a prior inconsistent statement or a contradiction:

(1) furnishing, promising, or offering — or accepting, promising to accept, or offering to accept — a valuable consideration in compromising or attempting to compromise the claim; and

(2) conduct or a statement made during compromise negotiations about the claim — except when offered in a criminal case and when the negotiations related to a claim by a public office in the exercise of its regulatory, investigative, or enforcement authority."

This pretty well directly contradicts your assertion that "[t]o make any offer to settle is an admission of guilt."  Of course, you'll probably say that you weren't speaking of admissibility and that you were simply saying that the other side would interpret an offer as an "admission of guilt."  In that regard, it's certainly possible that the other side might interpret it as such.  However, as "Tax Counsel" explained, no lawyer who knows what he/she is doing would view it that way.  And, in any event, if if the other side interprets it as such, it would be meaningless, because the only thing that counts is admissibility.