Thats true, but Dreamweaver wasn't really designed to view .tpl files in WYSIWYG (design) view.
I know you can make Dreamweaver MX view other types of unsupported extensions such as php4 as text, but it is specifically only designed to interpret HTML in design view. If your clever enough you might want to try looking into the resource XML files* and see if there is some kind of configuration which will allow you to insert the .tpl type of file for design view.
*Most of Dreameavers settings were done in XML, if you ca ntweak these, you might be able to modify the program to your needs. But if you don't know XML very well, I'd wouldn't recommend doing this.