A lot of people don't understand what tempered glass is.
When you temper a pane of glass, you are basically baking it. You heat it to a certain temperature for a certain amount of time, and then cool it slowly. What happens is the crystal structure rearranges itself so that the pane of glass becomes a single lattice structure. It is much stronger than standard glass, but when the panel fails, instead of cracking, the entire panel fails chatostrophically - it shatters into many small pieces (most of which are cubical instead of sharp and pointy). Tempered glass is used in many safety applications - particularly areas like car windows.
For aquariums, tempered glass panels allow you to build an aquarium with thinner glass. However because the tempering process is only afforable with large production runs, the glass panels are usually only available in limited sizes. Custom aquariums are almost always made with standard glass. Drilled tanks are always made with standard glass - even if it happens to be low iron glass like Starfire (or other brand names).
It should be noted that even though you can't drill tempered glass, you can drill standard glass... and then temper it. I am not aware of anyone who does this, but it can be done.