McTavish,
it isn't. However, being German (and I only mean this by terms of growing up there) you learn about the specifics of the legal environmen. We all know how difficult it is to ge aquainted to "common" law that everyone knows in the US yet are unique to the US.
i.e. In Germany no police would ever stop you on the Autobahn and just pull you over. Whilst they on rare occasions do this, it would always be to a parking lot for safty reasons.
With regards to the taxation, there is no difference as a citizen, however, if you are establishing a business in Germany many other aspects apply. i.e. IHK which is an organization entrusted by the Government to which you you have to pay dues for employent education regardless of the fact wether you have employees or not.
What I am trying to say here is not the issue of conducting business in a foreign country - enjoy doing it and trust me, you on't succeed (if you want to stay legal), the issue is that with all the paperwork involved as a sole owner of a company (incorporated or otherwise) you will ABSOLUTELY show enough CAUSE to USCIS that there is no intend to stay in the US as you conduct your business abroad and pay your taxes there.
Even if you have a 5 million home and all in the US won't matter as your financial obligations (property tax aside) do not reside in the US.
Further, you need to really figure out how VAT and all effects you. How you can declare that on a US tax return (remember VAT is something that doesn't exist in the EU and please do not try to educate me on this, VAT got demolished in Europe more that 30 years ago, the term is still used, but the actual meaning is different)
Yet, in the US (and Switzerland) you still have (in certain areas) VAT.
The whole issue of sending out invoices to customers (base for your taxes) and reconciling those with applicable taxation laws and the dues you have to pay to certain enteties in the area where you conduct your business becomes a total nightmare.
As I said earlier - whilst there may be no intent to break the law - anyone doing this will, unless they are total experts and then their line of business would be international accountant and I am yet to find one and my brother in law is German and US CPA and boy do we have fights.
Also, in Germany; as a regular employee there is no legal requirement to file a tax return since all your taxes are already paid out of your paycheck through your employer. Thus, if you have no other source of income, you don't file a tax return (unless you need to claim expenses)