Web startups typically didn't care that much if they lost say 1 out of every 10,000 transactions if that sacrifice kept them cheap and nimble. Since they usually didn't charge customers, losing 1 out of 10,000 is not that big of a deal. Of course, if a bank did that they'd be sued up the wazoo, and that's why transactional integrity and "big iron" database features matter to banks.