They hold on to any funds still in your balance for only as long as it takes to ensure you can cover chargebacks and reversals on your own payments, then disburse them to you and your relationship is ended. I don't think adding a dispute process to this would better the situation. It may be frustrating to have someone decide not to do business with you, but it's not illegal, and shouldn't be something you can dispute.
What I find interesting is that Paypal is essentially a bank, yet it's completely unregulated.
I've been through this myself. I had a merchant account about 8 years ago, underwritten by First National Bank of Omaha (the largest private bank in the United States). I got a couple large, unexpected chargebacks all at once from a single scammer that used multiple stolen cards. FNBO decided to create a reserve on my account for several days while they reviewed my account, then decided to terminate it due to the chargebacks. They held several thousand dollars of my money for exactly 180 days before returning it to me. That's money I needed to pay vendors and pay my bills - and it was perfectly legal for this regulated bank to hold it.
Thought experiment: why not make it 5 years, instead of just 180 days? If a 5-year term were to become "standard industry practice," to paraphrase Dan, who could argue otherwise?
They simply should not be able to exercise that kind of arbitrary power, at least not without being subject to standard banking regulations that can be modified as needed to balance the interests of financial institutions and their customers.
Thus illustrating one of my favorite pet sayings: "Any sufficiently dominant corporation is indistinguishable from a government."
Nothing you are saying actually makes this policy right; you are simply reiterating its existence.
We were investing in 60/90-day Brazilian T-bills at the time because that is where we were earning the money, so the interest earned was much higher than what you can currently earn on short term securities in the US, but when you are handling the quantities of money eBay is handling, the amount of interest that can be earned on 30/60/90-days securities is very very large.
This same approach applies to the money Groupon holds that belongs to small businesses. Whatever amount isn't being used to pay operating expenses is probably invested in some sort of short term securities.