More seriously:

- you get support by paying, this is important for many businesses - $1k/year is cheap

- risk of getting sued if the word gets out you're using something against its license (and for network-facing code, I'd suspect it's easy enough to miss something)

For me the advantage of source-available is you can always shortcut the support if there's a business critical problem and you can't wait for the author to wake up, so I think it's a great model.