It's pretty much a necessity for an online merchant to accept Paypal. Paypal knows this and takes full advantage.
When Paypal takes a payment, they either charge the purchaser by withdrawing funds from the purchaser's bank, which can be done immediately, or by charging the purchaser's credit card, which will get the money to Paypal the next day. Either way, they're getting their money within 24 hours.
They don't pay the merchant for 3-4 days, though. The extra 2-3 days of float makes them a little extra cash.
Paypal did $6.6 billion in transactions last year, or $18,082,190 per day. Assuming they get a 7% return on their money, that's .0192%, or $3471.78 per day, or $1.278 million a year.
Meanwhile, the merchants have to cover the cost of goods sold for 3-4 days until Paypal deposits the funds into the merchant's account.
Why don't things like this ever work the other way around?