Your notion of a user is based on yourself -someone who is both technology and IT savvy. For Symbian OS, the assumption is that the user may not have any previous computer experience. It's designed to support the technology savvy, but users as well for whom the phone is their first and perhaps only way of accessing the internet. Further, when things go wrong for that type of user, in incurrs support costs for the mobile operator. I've worked with operators who were limited by their service desk technology and simply couldn't expand their call centres further (they've since upgraded technology). The point being, that cost of smartphone support is actually the largest inhibitor to market growth.... and the growth of that market interests all commercial developers.
Manufacturer policy dictates whether an unsigned app. may be installed at all. Manufactuers may "lock down" phones to only signed apps. if they wish.
The reason for the differences between manufacturer and enterprise trusted access is that manufacturer access permits things like updating trusted root certificates (for app. signing), as well as OTA patching. These features, if abused, could compromise platform security for the device.
You're absolutely right that security need to be balanced with ease of use for the end-user... but you're misunderstanding who the "lowest common denominator" end user is from Symbian's perspective, and what that means to some of the security design imperitives for the platform.
You should also understand the high-wire act of balancing the interests of the operator (who's interest is revenue generation and minimizing cost of support) and the manufacturer, with those of the developer/enterprise/end-user. Symbian understands these issues better than any other platform provider.
Could the PKI and signing model have been better designed? Absolutely. I've never met anyone who has designed a PKI trust model who was totally happy with the result - and they're not easy to change!
If you're enrolled in the Symbian Preferred Partner, Nokia, or SE developer programmes, there are mechanisms for getting approvals for other signatures.
BTW, it really was envisioned that Enterprises would develop in Java. That any Enterprise has standardized on Symbian OS would come as a pleasant surprise to Symbian.