Whilst it may be frustrating for some not to be able to delete photos which you sync onto your device on the iPad itself, you need to remember it is a sync, and not an upload.
You can of course use iTunes on your PC and choose not to sync those particular photos to your iPad. The choice of what to sync is yours.
Use the "Photos" tab related to your device in iTunes on your PC, make sure only photos you want on your iPad are selected, and press the "sync" button.