Specifically, the FSF is urging "all computer makers implementing UEFI's so-called 'Secure Boot' to do it in a way that allows free software operating systems to be installed. To respect user freedom and truly protect user security, manufacturers must either allow computer owners to disable the boot restrictions, or provide a sure-fire way for them to install and run a free software operating system of their choice. We commit that we will neither purchase nor recommend computers that strip users of this critical freedom, and we will actively urge people in our communities to avoid such jailed systems."
The reasons for this petition, the FSF, explained is that since "Microsoft has announced that if computer makers wish to distribute machines with the Windows 8 compatibility logo, they will have to implement a measure called 'Secure Boot.' However, it is currently up for grabs whether this technology will live up to its name, or will instead earn the name Restricted Boot."
While admitting that "When done correctly, 'Secure Boot' is designed to protect against malware by preventing computers from loading unauthorized binary programs when booting." The FSG continued, "In practice, this means that computers implementing it won't boot unauthorized operating systems--including initially authorized systems that have been modified without being re-approved."
That's fine, as far as the FSF is concerned so "long as the user [is] able to authorize the programs she wants to use, so she can run free software written and modified by herself or people she trusts. However, we are concerned that Microsoft and hardware manufacturers will implement these boot restrictions in a way that will prevent users from booting anything other than Windows. In this case, we are better off calling the technology Restricted Boot, since such a requirement would be a disastrous restriction on computer users and not a security feature at all."