The reason is simple. The compacted foam that hit Columbia had a relative speed of 500 miles per hour. This is because when it detached from the liquid fuel tank, it rapidly decelerated in the atmosphere. The spacecraft continued at about the same speed (technically it was accelerating) and rammed the foam with a relative difference in speed of 500 miles per hour.
The tiles and carbon plates on the space shuttle are not weak. They only appear that way when something hits them at 500 miles per hour. A piece of plastic has trivial strength compared to the carbon plates or the heat shield tiles. It won't do a thing to help the spacecraft.
So if one came off today, one could come off later during re-entry, too.