What do you mean by "allow"? If the boot isn't restricted to signed OS X and Windows, you are "allowed". I've installed it easily on several older (2007-2010) Macs.
The reason they don't support it is likely because there's not the demand among their customer base, as most people who want a GNU/Linux OS on a laptop go for Thinkpad or Dell due to better compatibility and other reasons.
The reason they don't support it is likely because there's not the demand among their customer base, as most people who want a GNU/Linux OS on a laptop go for Thinkpad or Dell due to better compatibility and other reasons.