According to one book tearing paper towels on Shabbat is not allowed. However, tearing a tea bag to open it for use is permitted as long as one does not destroy any letters and as long as the tearing is done in a destructive way so no mending would be possible. Why doesn't the same reasoning imply that one should be able to tear paper towel as long one does not tear along the perforation?
You're on the right track, but there are some differences between paper towels and tea bags. Tea is food. You are allowed to tear open a package that contains food, so long as the action you do is not constructive (that is, you are not creating a new vessel by opening a sealed package). In the event that you are creating something new in the process of accessing the food, this action would be prohibited.
With paper towels, there is no such benefit. Destroying in order to create something (a pre-set paper towel) is prohibited biblically as an act of building (or a derivative thereof). Destroying for destruction's sake is prohibited by rabbinic injunction.
Along the lines you suggested, however, toilet paper may be torn on Shabbath (for its intended purpose) so long as one does not tear it on the perforation. In this way, one is not "building", and the rabbinic prohibition against destroying is waived for the sake of human dignity (ie., not to be forced to spend the day either in pain or covered in filth). But, again, no such leniency exists for (standard) paper towel use.