The entire concept of a Hotspot is for Internet Sharing. That's why, in Windows Phone 8.1, it is called "Internet Sharing" in the settings instead of Mobile Hotspot.
I agree that this should be possible to put the device into a Wi-Fi host mode for file sharing without the Data being turned on. With Microsoft being a "cloud-first" company these days, I suppose they think it is not possible that someone would want to keep their data off while turning on the mobile hotspot, because they think you should share files through OneDrive.
I'm not saying Microsoft is right for doing this. You asked, "Why is Microsoft resctricting us to do this?" and I am attempting to answer that very question.
The best thing you can do is to send feedback to Microsoft through the UserVoice program and request the feature. It is indeed a feature I would vote for. Hopefully, it is something we can at least get in Windows 10 for Mobile down the road.