Non-technical users can and will be tricked into doing all sorts of ridiculous things to their computer, and then they will blame the computer manufacturer for letting them do that. Computer manufacturers responded by not letting them do that.

Mainstream computers are designed for mainstream users... the common clay of the land... you know... morons. They have to be protected from doing stupid things to their computers (because otherwise that's how you get botnets).

As the OP says, there are computers that don't have these features, and that you can do whatever you like with. But they tend to cost more, in part because they're not mainstream so they don't get economies of scale.

That's true. However, vendors don't look at manufacturing costs in isolation - they care about profit. All these user-hostile additions generate more in profit than they cost in manufacturing.

This way, the best stuff costs more, even if it has less - because "value-add" garbage has negative total cost.

You're right that it is all about economies of scale. Economies of scale say that it doesn't make sense to tape out a whole separate die just for non-business consumers. They just don't sell that many units that it's worth it. It's cheaper to make one die for everyone and then sell one die with AMT turned off, even if it involves wasting a small amount of silicon for each chip produced.

Incidentally, this is why (apart from a few noxious exceptions like ECC) consumers are generally the beneficiaries of market segmentation. Businesses will pay a lot more, locking these features behind higher-priced models lets the consumer models be cheaper. Without market segmentation, the outcome isn't that you get a Xeon at the price of a Pentium, it's that you get a Xeon at the cost of a Xeon.

Maybe I'm missing something here but how is it possible that on-die features like IME affect the manufacturing cost and complexity of a laptop for e.g. Dell?