Why do speakers recommend amp wattage?

Probably a rough estimate of the power needed to drive the speaker to max. sound pressure level for extended periods of time over the full frequency band for most recorded music. Amps that are underpowered may not be able to produce enought power at peaks resulting in clipping and speaker damage.

A 200W amp will be okay, not much chance for speaker damage.

A 7w amp might be okay if you don't play very loud (but SETs are effected by so many other factors, so I would avoid using them unless you are really shure)

Probably because people expect it. In one sense it isn't very useful since size of room, average listening levels, type of music, ability of a given amp to driven a given load, as well as other factors play into how much power the amp should have. However, it gives you a general idea where to start. This is an area where there are so many variables that absolutes can't be given.

BTW that 80W is "power handling" which is different than recommended as it is the maximum they recommend. Probably the maximum continuous which is very different than the maximum peaks when playing music. Given their efficiency of 87 dB I think you would want an amp that is at least 80 watts and more wouldn't hurt.

You could put a kilowatt in front of your Ushers without fear as long as you were actually listening as you turned it up. Running at 'redline' for extended periods will probably result in some kind of meltdown, or ear damage!
Low power at too high a level is probably worse, as noted above.

.

Tradition, I suspect. Often, the impedence curve is more important.