Is this some kind of religious belief conflict?
Basically, it's the fanboi thing.
The wife and I started out with purchasing an IBM XT clone running DOS 3.1. I then built many of my own PCs over the years and switched to Macs when I started my own prepress business. That because almost that entire industry is built around Macs. Or at least, was. More shops are becoming Windows based doing the same work, but it's still heavily Mac dominated. It was very important at the time to be on Macs since Type 1 PostScript fonts were (and still aren't) transferable between Macs and PCs. You can purchase the supposedly exact same T1 PS font from Adobe, Linotype, etc. for each platform. But, even from the same vendor, type reflow isn't quite the same. So, you have to carry a project through from start to finish the way it was sent from the client. And that generally meant, Mac.
All of that boils down to, I have no real allegiance. I've used both platforms for many years, and am comfortable with either. I prefer Macs, but that doesn't mean I won't use Windows.
My other theory (notice it's almost always new users), is that they think they're making some sort of threat to Apple employees that they're going to leave and get the "enemy's" product. Well, everyone here (as I believe you already know) are other users. The person wants to ditch their Apple device? Go ahead. It's their money to spend as they wish. I have no incentive to try and talk them out of it. But the way they state their intentions leads you to think you'll lavish them with a gift or something to keep them in the fold.