I don't know if "intended" is the right word, but it is certainly not a bug. The reason you don't see previews is because the images are not on your machine. In order to make Drive work and to allow it to keep up with the ever-changing nature of your Box content, they simply show you pointers to your files, not the files themselves. The reason you see the thumbnail of an image if you open it is because that image is not in the local Box Drive cache folder. Eventually that will get pushed out of the cache folder and you will no longer have the preview.
Box has certainly heard this request and are brainstorming on ways they could do it, but I'm guessing that most users would vote for speed and currency of information over image previews. I think if Box can deliver it all, they will do so. Bear in mind that Box Drive is still in beta and even after its full release not all of the desired functionality. If they waited until everything was there, we'd never get it. I want preview as well, but there are other features that I need them to have before I can promote it to my users. I'd much rather they spend time on basics like file locking before they work to solve the preview issue.
So it's not you or your machine. It's just something lower down the priority list. Depending on our line of work it could be a showstopper. I hope it's not, but if it is, you can just forget Drive for a bit and come back when they've got it.