1) They enjoy paying $3000 for a laptop;
2) They heard somewhere that macs are better for multimedia applications (Please note that it is never a good idea to confront a macfag on this matter. They will not be able to produce any reason other then "because they are" and your head will explode.);
3) They think it will impress people at the coffee shop;
4) They enjoy being different;
5) They scoff at the idea of compatible software;
6) Because macs are available in pretty pastels.
Steve: "Nah that place is full of hipsters and macfags."
They are mainly found on YouTube making reviews of mainly useless or unaffordable products (see tat) discussing Apple "rumours", providing viewers with "giveaways", giving updates on their boring lives, instructing viewers to follow their boring lives on Twitter and discussing how terrible a PC running Microsoft Windows is and why Macs are better in every way.
This term is not to be confused with someone who knows what they're talking about and has plenty of experience on all sides of the table, i.e. someone who is not a noob, but uses a Mac.
Non-noob viewer: Macfag, shut the fuck up.
These people often drive Priuses, wear uber-nerdy close on purpose, hang out in coffee shops just to be seen, tell you how much they 'care' about the latest human rights/animal rights craze (Japan, Haiti, etc.) and always offer their unwelcome, obnoxious opinion no matter what. These tools are walking Mac commercials.
Mac fag: "Hey, I took some pictures of the crash."
Level-headed person: "Thanks, man, that's helpful."
Mac fag: "Check it out, if you do this with your finger you can zoom in for more detail and did you know that my iPhone 4 can even *blah blah blah* five minutes later You know the Snow Leopard OS doesn't have all the virus and security problems that a PC *blah blah blah*...
Level-headed person: "Dude, seriously. You're such a mac fag. It's really not a good time, and my PC is just fine. Stop preaching at me. There are more important things to worry about...like this fucking car accident you caused."
Mac fag: "I get way more miles to the gallon with my hybrid and did you know that I can check traffic on my iPho-- *punch*
Everyone else at the scene of the accident: *applause*