It's in The Thousand-Year Door, too. Sure, the Piders shoot the web balls out of their mouths, but...where do you think the web they're hanging on is coming from?
Also, in real life, spiders who spin webs shoot the string from their butts. I think silkworms, who we get silk from, do the same thing, so if you wear silk, congrats! You're wearing caterpillar poop.
On top of it being reality, look at their introduction in NSMB. Notice their position when falling from a string. Guess which end they're hanging from.
<Adam> I feel like smwc is a prostitute now, because we put up a porn ad for money