How/why is that?

Let's say that the legal entity is registered in Switzerland or China or Russia for that matter.

This company sells some goods [virtual/physical] through a web site [for example using Amazon 1-Click patent] to a person, who happens to be an American citizen, with payment made by credit card.

How this company will become "subject to these rules and ridiculous software patents"? In which court will this case be processed? How the court decision will be enforced?

Citizenship doesn't matter, but the location of the user does.

If you're doing business with American users, defined as "people residing in or doing business within the borders of" the United States, then you're subject to the patent laws.

All a claimant needs to do is prove that this is the case and the lawsuit can proceed. Then you'll need to find representation in a particular court in Texas to defend yourself, or you'll have to settle out of court at your own considerable expense.

You might be subject, but you won't have a judgement enforced against you.

If you're "doing business" in the United States, you're subject to those laws. This is just how it works.

If you're sued and you don't defend yourself, you will probably lose by default, so you can have a judgement enforced against you. This might preclude you from travelling in to or even through the United States.

If that last part is the only thing they can do, I say move to somewhere warm and cheap and just "agree" to their terms ;) I thought they could call InterPol on you or something...