Or, investors do understand the difference, but think they're buying low to sell high to "greater fools."

If the market can remain irrational longer than a fundamentals-driven investor can remain solvent, is it irrational to bet on the market remaining irrational?

I have seen both. I have heard some talking on live television about how it is a bubble, but it isn’t popping yet, so keep investing. Everyone seems to just be trying to get theirs while the party is still happening.