[daemon13]

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?

[astrodust]

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.