Most of us, quite rightly, react poorly to an ultimatum. That's because an ultimatum is an emotional affront, a deliberate break in a relationship. Do this or else!
Often, our instinct is to respond to confrontation with confrontation. Ultimatums rarely work because we react to the emotion instead of responding intelligently.
On the other hand, giving your partner in a negotiation or a sale a choice between two outcomes is a generous act, a form of truthtelling that helps both of you. We all make choices, and choices have consequences. Helping people understand them in advance leads to better decisions.