You can always negotiate, but if you accept an offer I expect you to commit.
Doesn't this go both ways? In this day and age in America, there's no loyalty going in either direction, employer or employee.
"Make game of that which makes as much of thee."
Some people steal; some might have even stole something from you. Doesn't mean you have to too, and I doubt most people are held back only by law/punishment.
I think it's hard for most jobs, at least in my space (mobile software) to do much effective work when staying only for -let's say- 3 months at some company.
But mismanagement happens. The example you mention of being let go after just 3 months should be the exception, not the rule. I certainly never experienced something like that. Then again I like to work for big clients, feels to me more guaranteed they will still be in business tomorrow and me getting paid.
Of course, once the candidate joins the company, regular interpersonal concepts between employees that involve loyalty and commitment can be pursued.
To all the people mentioning ethics - a company can and will fire people without any reason. I know a couple of people personally who can't sleep not knowing why they were fired. In 30 mins, they were walked out. That's incredibly unethical but for some reason, not giving advanced notice to your employer is considered unethical? Hypocrisy!