Clinton,
You are looking at this issue in a far too simple way. Firstly, immigration from one country to another is not an easy subject to explain in a general way. Every country has their own rules and can do what ever they want, regardless of what passport you happen to have.
So, just because you may have a UK Passport and also don't have a criminal record does not mean the Americans have no reasonable reason to stop you working in their country.
Essentially if have a particular passport, you can work in that country. If you have an EU member passport, you can work anywhere within the EU. Anywhere else, you are going to require a work permit at the very least. To get a work permit, generally, you need to be married to a citizen of that country, have a parent who is a citizen of that country or there has to be a local need for someone with your skill set.
That's about as simply as I can explain it.
Last edited by Kenny; 2nd Feb 2012 at 07:14.