Oh, I agree wholeheartedly; however, the fact that it doesn't always go that way is enough to render a lot of criticism about US-defeating TLs null.
Besides, a lot of these PODs (and even invasions) occur at a time when US industry was pathetically inferior compared to Europe, and the US armed forces even more so. Take Germany vs America in the late 1800s/early 1900s. A lot of people here invariably say that US would win because the Germans couldn't invade; well, the US was able to invade. And it was far weaker than Germany economically, industrially, and militarily. The High Seas Fleet was strong enough to pound the US navy and also any shipbuilding facilities on the east coast (thus delaying shipbuilding), and with control of the seas, it could land an army. But responses to this one were wide, using ideas and actions that people just didn't do at the time (for example, American partisans; this just wasn't a concept in war yet. The francs-tireur were minor, very very minor, and notice that occupied France + Belgium didn't give Germany anything that can be considered "trouble" in WWI). But people just shrug and say "couldn't have happened," which really gets my goat.