In the absence of a "socialist" state, US colleges provide poorer students with a free/subsidized education by charging richer students more
True for a lot of places. Last I heard you could get Harvard for free if your parents make less than some reasonably large amount ($70K or so)
US faculty receive massively higher salaries.
True, especially compared to the UK. There is in general a lot more money floating around the US university system to make life easier. My officemate recently moved from a postdoc position here at Big-Name American University to a lectureship at Mid-Ranked UK University and was bitching about how he lectures two and a half courses per semester (as opposed to the usual one course in the US) and has to do ridiculous things like taking out his own garbage because the university won't pay for cleaners.
In a country where college education is commonly used as a mark of social status, they can charge a premium and people will still pay
Well if we're comparing to the UK, then the UK is if anything more concerned with the status conveyed by education. The British elite will judge you not only by your university but by your high school.
The main reason, in the end, is that the US is the only major country where the higher education system isn't primarily an arm of the government. Great private universities just don't exist in other countries. There are upsides and downsides to this -- the upside is that America has most of the best universities in the world (can you imagine if it didn't?) and the downside is that they can be pretty darn expensive.