I believe the logic is that it makes taxation something explicit that you are forced to notice. The idea being that an included tax would be invisible and allow for hidden changes and less recognition of the government's role.
This misses the point though. The problem exists at any tax rate greater than zero. Doesn't matter if it's a penny or 100% of the price, the issue is that the advertised price is not the monetary value that's expected to be exchanged for that product/service.