Actually, the kilogram is supposed to be a standard mass rather than weight, if you can believe that. Thus in a weightless environment, it’s supposed to still consist of one kilogram of mass although weighing nothing. (The English System equivalent is the slug, which on Earth weighs 32 pounds; weight is a force, specifically the force exerted on a mass by gravity.) Problem with mass is that the only way to keep it constant is to guarantee that your original standard mass still has all of its atoms from the time it was forged and standardized (as it were), and there’s no way to do that.
And yes, if the standard is off, then all derivative standards are off.
In the USA, it’s up to Congress to set standards of weights and measures (per Article 1 section 8 paragraph 5 of the Constitution); that’s the main reason why we haven’t adopted a foreign system such as the so-called “Système Internationale”.
No, really it's the (understandable) stubbornness of the people. We don't want SI, so we don't use it. Except 2 liters of pop. Cause that's bigger than 2 quarts.