I think it would be hard in practice to allow root access but limit it to people who know what they are doing and understand the risk. People will follow arbitrary instructions to modify settings and enter commands (that they don't understand) for the most trivial things and from the most untrusted groups (for example: to install pirated software, to customise their user interface, etc.), and will merrily click past every warning without reading it.

As a result, there will be more malware if you even make enabling root access an option. It isn't surprising that Apple and others are happy just not catering to that small segment of the market.