Slap me around and call me Sally, but having sold over 10,000 items on eBay (not that it's particularly impressive, but lets you know I have some experience) there are two reasons why I would withhold feedback:
1. I used to leave feedback first. Then I found out that only about half my buyers were returning the favor. On average, I had around 800 people I was waiting to get feedback from in any given 90-day span.
2. One buyer ruins it for everybody else. I ship the item the same day I get payment. Something goes wrong (either the USPS loses it (consistently lost 1 in 200 packages), or items break during shipment, or item is not what customer expected, whatever...) Rather than contact me for a solution, they leave me crappy feedback.
Withholding feedback gives a buyer incentive to write you and report the problem before giving you negative feedback. This allowed me to correct any issues (which were almost never my fault) and satisfy my customer. About half my business was repeat, so I musta been doing something right.
However, I admit a lot of eBay sellers' intentions are... less honorable.
Ok, you may now call me "Sally"... *slap*