So why not offer $200K and require proof of currently working at X?
Many people in the startup world don't understand this, but the vast majority of people in the labor force are not primarily motivated by money. You need to pay them enough to feel like they're not being taken advantage of, but beyond that, they go for work environment, interesting coworkers, challenging projects, and other intangibles. To have any chance at all of hiring them, you need to provide those and not just money.
(Google understands this very well - they explicitly state with their offer that most people who work for Google do not do so for financial gain. They do it because they want to be a part of something great, and have really intelligent coworkers, and be given a flexible and creative work environment. Yahoo has a big challenge matching this, given their current lackluster stable of products, and Marissa's trying to jump-start the virtuous cycle and bring in folks that people would want to work with.)
False dichotomy if there ever was one.
That seems at odds with the fact that most of these companies products are immediately shut down when the acquisition is announced.
It's often a bait-and-switch tactic on the part of the acquirer. Once the deal is complete, the acquirer owns everything, product and all, and has no legal obligation to keep it open and active. If you wait a month or two after the deal closes to announce this, then many of the employees already have large sunk costs in the employer - they've possibly relocated, they've signed up for health insurance again, they don't want to have a tenure of 1-2 months on their resume, they've learned some of the acquirer's systems, they've made contacts in their new employer - and so you can keep them even if it's not a choice they would've made initially.
Besides, very few companies are transparent with salaries. If yours is not, that means that salary negotiation is not always fair to existing employees and they don't want you to know that. (It could also mean salary negotiation is unfair to the new hire as well.)
All of these parties come into the mix in an acquihire.
The investors have preferred shares, quite often with a liquidation preference, and they will always get something. The founders will often get something, (though in the case of liquidation preferences, it may not be very much - "consulting bonus" is not uncommon here) in an acquhire. The employees of an acquihired may or may not get totally screwed on their common shares depending on the size of the liquidation preference of the investors. They will almost always get a "retention bonus" - that may be on the order of a 50% of their salary, repayable if they don't stick around for 1-2 years.
All companies I've worked with have been pretty good about leveling salaries, and are careful to avoid a situation in which you have two groups of people doing the same job, but one group making 50% more than another. You may have isolated situations in which an employee or two is out of wack on their compensation, but companies like Yahoo! try hard to avoid having systemic disfunction in compensation. The mechanisms for getting new employees in a tight job market come in the form of hiring bonuses and/or acquihires.
Reading your comment carefully, I think you are considering the scenario of an acquhired where most of the employees acquired were founders, whereas I'm thinking of the 30-50 employee company, where only a couple of the employees were founders.