A raw file usually has two previews embedded in it, a tiny thumbnail preview and a larger zoomable preview. It sounds like LR is only able to retrieve the tiny thumbnail preview even though you are able to zoom in on previews in the camera. Unless the camera, itself, is actually decoding the raw file to produce the zoomable preview and none is in the raw file for LR to see.
You can use the following website to investigate what previews are available in a raw file: http://regex.info/exif.cgi You want to click the Browse button next to Local Image File and navigate to a raw file and click Ok, then click the View Image From File button, then wait for the raw file to be uploaded, after which information about the raw file will be shown including the sizes of the previews and raw image.
As to your question about whether using a card reader might help, try it and see. There could be a system driver getting in the way when the camera, itself, is plugged in.
Checked image on that page and shows all different sizes of previews. Looks it's a problem with lightroom itself.
Tried to import manually to another location and import pictures as local files - previews showed correctly. I found similar issue discussed a while ago with nikon camera too: No thumbnails on import [Archive] - Lightroom Forums
So I guess the only proper solution for now is to buy card reader.