Just as a thought experiment if every piece of software had a similar clause in their license it would mark the end of software patents. Isn't this what we actually want more of? Isn't _that_ actually the moral high ground?
If that were Facebook's goal, why wouldn't they just team up with a few other large companies and lobby for abolishing software patents in the US?
I actually think this is Facebook's goal. As for your question, I think they aren't trying to abolish software patents in the US because they judge (IMO correctly) that it is unlikely to succeed.
That seems highly unlikely, considering the way Facebook operates.