The problem is not that enthusiasts who want to run an open source operating system won't be able to choose appropriate hardware to run that OS. The problem is that Secure Boot may remove the option of buying a Windows PC and then at some later time deciding that you want to run an open source operating system on it from those who weren't thinking about open source operating systems to begin with.

At the moment, every PC sold is a potential target for a Linux distribution or other open source operating system, and hence every PC sold increases the size of the potential market. Secure Boot has the potential to disrupt this: a PC with Secure Boot may deny the owner the ability to install and run an unauthorized operating system. This could significantly reduce the potential market for open source operating systems.

Look at it this way: at the moment if your mum/dad/grandparent/whoever is willing to try a different operating environment, you can install pretty much any Linux distribution (or other open source operating system) on their hardware, or if they can do so themselves. And by 'can' here, I don't just mean that you personally are able, but also that the original manufacturer allows that kind of thing to happen.

In the future, computer manufacturers may sell computers to people on which you (or they) will no longer be allowed to install an open source operating system for them to try out, even if they want to.