So to expand on what ajejee said above:
Previously, Cerberus could only be installed either as a regular app with the "Device Administrator" permissions like any other security app, and the associated flaws (like not being able to turn on GPS on 2.3.3+), or by downloading a separate APK off the website and running it on your phone, which would copy the APK to the system partition.
Long story short, putting Cerberus in ROM Manager is a way to make it easier for users to install Cerberus to /system, since most competent root users have ClockworkMod Recovery and at least know of ROM Manager. While yes, it is "odd" for an app to be in the ROM listing, it's not the only non-ROM in there. I also have radio images for the HTC EVO 4G listed as well.
In regards to placing it in "Extras": Yes, that's a nice idea, but with the way RM's manifests work, it's not really feasible. The "Extras" isn't a separate category, but just another manifest that was hosted on Koush's GitHub, and was only being used by Superuser. This means that any time Superuser had an update, or Cerberus would have an update, the developers would have to send a pull request to Koush, and someone would have to merge it in.
I say was, because Superuser and Cerberus both now have their own separate manifests (meaning no more Extras link), and show up as "ROMs" in the Download ROM menu. I've talked to Koush about making a separate "Download App" menu, but the problem with that is that the Market's ToS prevent having an app that just downloads other apps.
tl;dr, this is so Cerberus can be protected against factory resets, apps can kinda-sorta go in the Download ROM menu, and ROM Manager can't have a separate Download App menu.