are trying to fool the populous by making a letter "look" important, but read the stamp area. Even if it looks like a stamp, many times it's not and a cleaver ploy to make the recipient believe it is 1st class and not "bulk" or "presort standard", which are both junk or ads.
We had found in our mail operations that if we didn't cross out address and any bar codes on the front and back of 1st class mail, it would often return to us once again. It could be different now and you only have to put "moved". Just our experience at that time.