It's a design decision, not a tax/regulatory issue. We've had one-off tipjars for decades and they haven't changed the game. My mortgage is recurring so my income needs to be recurring as well.

We currently have an "Other Ways to Give" on your profile where you can link a bitcoin address or Venmo account. That's an outlet for one-offs. We may mix in first-class one-offs eventually but we're not in a rush. See https://github.com/gittip/www.gittip.com/issues/5.