The US should cut off all foreign aid to countries that say Zed, and make them say Zee if they want money from the US. Then, systematically, we can use those countries as a base of support to require the rest of the world to say Zee. Zed is too cumbersome to say and sounds weird.
We should probably consider using the metric system, but that's another story. At this point, saying 'zee' rather than 'zed' is firmly established in American English. That's what pre-schoolers learn, that's how all our talking toys sound etc. To make that much change at this point in time, doesn't really seem worth it.
Many Brits express the opinion that being British means that they are automatically experts on the linguistic history of their own country and that Americans are always (or almost always) wrong in any opinion they express. All one needs to do is google a question like "why do Americans say so and so when British say a different so and so? I'm not saying one pronunciation is better or worse than the other; that kind of argument is just silly. People say things the way they say them because of what they're used to. We should consider ourselves lucky to be able to communicate easily with such a large portion of the world and not squabble about petty differences.
The United States does not need to adopt British English. Americanized English is just fine, it serves its purpose here. Each country can have its own take on the languages spoken. Not all English-speaking countries need to adopt the exact same British English standards - just like other countries don't have to adopt American standards.
Why would we change the way we speak and pronounce things? Just because another country (or many other countries) say things differently than we do here in America doesn't mean we have to completely change our language. Having a distinct way of speaking English is just another way for our country to be unique.