The process of verifying whether a script was coded by the OP or not, is easier compared to a DB. Ask the guy selling the script to make a structural change, temporarily. If he can do it, and knows his way in and out of the code, then it's most likely his. People looking to con other people by selling a stolen script most likely won't have any idea of the code structure within.

A theme is not a database; granted it's a digital good but you cannot put a theme and a database in the same category. A database is much more than just a skeleton layout for a website; it's content. Having the same database as 10 other sites out there is a significant risk to not only the site owner, but to the members in the database. Privacy, for example (emails), is a major concern. Obtaining a hacked database is far easier than obtaining a stolen script and/or design. Verifying whether a script or a theme is original, again, is far easier than a database.

Database sales were allowed, and then later disallowed because of the problems caused. Scripts/themes didn't result in as much headache as the database problems did, so they're still allowed and not banned. Go figure.

The same is the case with account vouchers/premium accounts; their origin is questionable, at best; you cannot verify their authenticity with as much ease as you can a script or a theme's origin, and they caused a lot of trouble in the past which is why they were disallowed.

Logically, there's no reason to disallow the sale of a database, but when you look at the problems and issues involved, it's just far easier to disallow them entirely then to deal with the crap involved in policing and verifying those sales.