This is absolutely true. And not uniquely American. Iceland has the same problem - many of the major sites are right off one of the various major roads (Golden Circle and Route 1/Ring Road). I visited in January and naively thought it would be quiet. Ha! I was so wrong. Reykjavik, Golden Circle, and the southern Ring Road area (out to Vik) were teeming with busloads of tourists. Trash, trampled muddy paths, and idiot tourists ignoring safety signs. We found it best to leave the house well before sunrise to beat the busses to wherever we were going that day.