Are there accurate meat scales which interface easily with a computer?
What would a network-attached scale even do?  You'd have to configure it to report to something, or have something else poll for it (and then you'd still have to configure it, either via DHCP, BOOTP or similar to set its IP address).  It'd be a security nightmare, as it'd be like a network attached medical device (or old printer) that never gets updates to deal with security vulnerabilities.  Some sort of a serial connection like RS-232 or USB is just plain safer in this situation.

... and there are plenty of USB digital scales, as they're used by companies to weight packages &amp; print out postage.

The only reason to not go with USB might be distance limitations (5m for USB 2), but you can get USB over Cat5 'extenders'.  (which would then meet your 'cat5' cable requirement ... but you can also get DB9 (or DB25) to RJ45 adaptors, if you just want to run 232 over cat5)

(I'm a programmer/sysadmin at a US government agency so it's possible that my security concerns are a little higher than most other people's.  Even if you keep it on an isolated network to only talk to the terminal, odds are that the terminal is going to need to be updated at some point, and you risk it getting infected.  Even if they exist, it's just not worth the trouble.  And yes, I do know about the attacks via USB devices)