It's the same thing as plugins, unsigned drivers, etc. etc. If the platform gives unfettered control, that is nice in that it gives power users flexibility, but

It's the same thing as plugins, unsigned drivers, etc. etc. If the platform gives unfettered control, that is nice in that it gives power users flexibility, but it comes at the cost of seriously jeopardizing the experience of non-power-users who don't understand the intricacies of the software. They expect the platform to protect them by default. This applies to the whole package -- security, performance, and overall experience. Vendors have learned that if they fail to provide this protection, the average user will blame them directly, not the extension that makes memory usage bloat 6x. The solution is to enforce stringent controls, including limiting modifiable parts of the experience to some well-defined areas.