I also experience this issue. I believe it is because the material is using a proxy to adjust its opacity based on the player's distance. Hammer doesn't work well with texture transforms in VMTs, as evidenced by the strange rotation phenomenon if you happen to use the security glass texture anywhere in your map.
The reason you are able to see a preview of the texture at all in the materials browser is because of the special preview image that A Boojum Snark included in his mapping resource pack. This is one of the many improvements to the material browser that assist the mapper in finding materials that are hidden or obscured in the default TF2 Hammer setup.
If you would like to see the texture in Hammer's 3D viewport, to make it easier to adjust its size and position, change the viewport's mode to 3D Textured. This can be done by clicking the little 'camera' button in its top left. This also takes care of the security glass issue and allows you to see envmap reflections in reflective surfaces such as window glass.