I'm curious whether this applies to personal towards business accounts. I'm not clear why Paypal would/should care if you send money every month (say, $10k) from your own account to a corporate merchant account.

The benefits are huge - if you have a points multiplier on your card, say 1.5x, the txn fees are 2.2%, you are looking at ~$1,350 to get 100k points - which is nothing to sneer at.