I suspect this happened because when DELL first launched laptops with Linux, it gave them some surprising bad press. Certainly the most famous being the woman who apparently was dropping out of college as a result of accidentally ordering a Ubuntu DELL laptop.
https://www.youtube.com/watch?v=5Qj8p-PEwbI
This probably seems like a weird one-off, but this story went pretty viral, of course it went viral on the internet for other reasons, but I'm sure the blame being placed squarely on DELL probably did lead to them being a bit spooked about consumer confusion.
The fact that nobody at DELL or elsewhere could just tell her that she can in fact, pretty easily install Windows on the machine is... well, a little frustrating to say the least. The college's IT department was aware enough of Linux to be able to say they can handle it, but not enough to say "Y'know, just for what it's worth... You can just buy and install Windows."