They have all your saved passwords from your web browser, too, and your mac user account's password. I'd guess there's a reasonable amount of people who re-use at least one of those as their 1password master password.
They're betting on a future weakness being discovered. Sure, it costs them some storage space to sit on these databases in the short run, but if they do get cracked, they hit the absolute motherlode.
I wonder if that's speculative - or if offline bruteforcing them works often enough for it to be worthwhile for the malware authors?