The CST-100 heat shield is replaced each flight because it is discarded as part of the landing process.
The capsule has airbags, between the heat shield and bottom of the capsule. As the parachutes slow it down for landing, it discards the heat shield, inflates the airbags, and lands on the ground.
Thus they can never reuse a heat shield, they throw it away on each flight.
Dragon V2 on the other hand will start by landing under parachutes in the water, then under parachutes on land with minor SuperDraco firings, and finally fully powered on land with the other two approaches as backup modes.
SpaceX has strongly suggested that the heat shield will be reusable but of course they have yet to demonstrate that by reflying a used Dragon V1 capsule. They have spent the engineering resources to prove it, and they have 7 recovered heat shields to examine, so they likely have very good data on the issue. Thus they should be able to have a confidence level that it will work but the proof is in the pudding. Lets see one fly 2 or 3 times and how they stand up at that point.