This is an oversimplification, but everybody who owns an Apple anything, is convinced that Apple viruses, and thus exploits can't and therefore don't exist.The point isn't that Microsoft allows customization, it's that Mac doesn't. You literally have to hack into the operating system. You're exploiting a new hole in their code every time they patch it too, and if you can get in, so can any jackoff who also knows a thing or two about writing viruses. And causing driver instability because you don't like having the default OS is going to cause even bigger problems.
The introduction of fear into a population is a time honored tradition in the field of propaganda.
Doesn't M$ know anything about this subject? Yet as you say, they allow customization. Can happen and will happen are two entirely different things. At least some part of the message from Apple here is, that they believe their product is so superior it can't be improved upon, and you're only allowed to have what Steve Jobs says you are, Now pass them your checkbook, please.