The point is, you shouldn't need to root the device. That's a red herring. You can completely modify the source and then flash the device. All these manufacturers give you methods for flashing. You mention Sony[1], Samsung[2], HTC[3], here you go ... In time projects like Cyanogenmod will have ROMs baked for you that'll give you root. But why should it be easy for a user to root their device? If the user can do it a malicious program can do it. There is nothing wrong with extra security so long as the platform stays relatively open, which it seems to be. If anything handset manufacturers seem to be getting more not less dev friendly. Would you want Windows or MacOSX easy to root?
[1] Sony Android source based on AOSP (Android Open Source Project)
You need root, because there are apps you want to run, that need root permissions. The use cases for these apps are very wide, from ad blocking to repairing OEM fuckups (I'm looking at you, Asus).
Anyway, it is easy to "root" Windows or Mac OSX. It is just logging in as administrator in Windows or using sudo in OSX. The paranoia and preventing the user to really control the device they bought is a new thing, that came with mobile OSes. It wasn't so widespread before.
[1] Sony Android source based on AOSP (Android Open Source Project)
http://developer.sonymobile.com/knowledge-base/open-source/a...
[2] Samsung Android source n stuff (unfriendly interface, a little digging is necessary, Samsung's got a lot of products)
http://opensource.samsung.com/
[3] HTC Android source n stuff, "Kernel Source Code, Binaries and Updates for HTC Android Phones"
http://www.htcdev.com/devcenter/downloads
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Google's AOSP n drivers (for completeness)
http://source.android.com/ https://developers.google.com/android/nexus/drivers