Also, there are "hackable" products on the market, e.g. the OpenWRT routers. Those still seem to be allowed.
OpenWRT routers have been in the legal clear zone for a while (due to FCC exception for the WiFi bands), but that's changing [1]
[1] http://www.cnx-software.com/2015/07/27/new-fcc-rules-may-pre... (FCC page https://apps.fcc.gov/oetcf/kdb/forms/FTSSearchResultPage.cfm... )
People are somewhat confused about this. For instance you can change the firmware of smartphones, no ? Well, no, you can't. You see, what people call a cell phone contains a 100% locked down actual cell phone, and a PDA that's completely independent (and in some cases, more than one locked down system. E.g. NFC is it's own locked down system as well). These are fun things, they're smaller and smaller. In the last 2 years the actual cellphone, everything except the antenna fits in 0.5cm x 0.5cm x 0.1cm package that actually contains multiple chips. These things are connected to the audio devices of the phone, and therefore have sole control over what comes out of the speaker and where the microphone goes. They also have a serial network interface these days for 3G/4G, and the sim connects directly to these chips.
So technically android (or ios, or windows phone, or blackbery, ...) does not run on cellphones, it runs on the PDAs that we insert into cellphones. Therefore it does not need approval.
It also means that Android/IOS do not actually control (for example) the audio on smartphones, or whether a call is answered. Or whether, when asked for the location of the phone or the contents of SMS'es, or to call out and send out the audio the microphone receives, whether those requests are answered. Android/ios would never see those requests (though you may find the microphone suddenly doesn't work anymore in apps). And yes, this works even when there is no sim in the phone.
Whilst there are cellphones that don't provide law enforcement (I wonder if it's only law enforcement though) with these features, they're older models and are getting phased out. If you read this site, I bet your phone has these features. And like all features you don't control, you wouldn't like them if you knew what they were.
Essentially, the law demands that no matter what you do with the software, the product won't malfunction (not just "not explode", it needs to work). Obviously that means no software that isn't made by the same engineering team ... also once the hardware is out that means no changes to the software unless someone does make their fridge explode. Even if they change the temperature 50 times a minute with the door open that can't affect the lifetime of the product. Needless to say, you block this in the software.
Unless courts start putting responsibility for the things people do with their own hardware on those people, this won't be forthcoming.