For me, because of my mental exhaustion[1], reading threads on twitter is also labor, so I feel like reading + replying + retweeting is my contribution back to their conversation, without being asked for $ on top.

Plus, because they're often using paypal, by the time you deduct paypal fees that the recipient has to pay, to get a meaningful amount, you'd have to donate the cost of an ebook. And a whole ebook is CLEARLY more work than 20 tweets!

[1] Chronic fatigue syndrome + pain meds.