I think you missed the point that we are talkinig about radio.
When people fill a page with noise on Slashdot, they aren't really using up a scarce resource. Slashdot would just get more servers if they ran out of bandwidth to present blather to readers. So, the only thing that's really being wasted is the reader's time, and the reader has mechanisms to avoid that such as moderation, and I think "foe" lists (I haven't tried them).
On radio, in contrast, frequencies in which to operate are a scarce resource. So, that noise is getting in the way of a more useful communication. And while we can tune off the channel, we don't have an infinite supply of other channels to use.
The situation is made worse by radio propogation, which makes many of the frequencies we do have unusable for much of the time; by issues like the hidden-transmitter problem, which make frequencies that might appear usable by one station unusable by the one he's trying to talk with; and by various incompatible sharing partners, the worst being PAVE PAWS out here in California. So, frequencies in which you can do something useful become scarce.
So, we have valid reasons to keep as much noise as possible off of the Amateur bands.