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Generally, beta is supposed to be almost completely identical to the release version, to ensure that what gets shipped to release users is tested. This particular pref seems harmless, but you never know.

Developer edition is what used to be known as "Aurora", which is in between Beta and Nightly.

I think they want to encourage wider adoption of the Beta version, so they treat it similarly to the Release. The logic seems pretty questionable to me though. If you can install a Beta version of Firefox, you should be able to avoid consenting to allow malware to run on your computer (this change is primarily targeted at extensions installed by some user action -- like something silently side-loaded by an application installer).

I don't see anything about why the beta, but I did see the following in the FAQ. Maybe this will help:

"There will also be special unbranded versions of Release and Beta that will have this setting, so that add-on developers can work on their add-ons without having to sign every build."