I bought 1 (ONE) unit and brought it in from SG and paid customs tax.

You can attempt to not declare it, and might get away with it if they don't perform car search

As for stuff coming thru KLIA... sorry la you can't escape.

Technically you are correct.

However they are not going to send an officer to follow you around and make sure you don't sell, so they take the 'worst' case scenario.
Duty free doesn't mean free of customs tax at the arrival point ok? It means free of tax from the country of purchase.

http://en.wikipedia.org/wiki/Duty-free_shop <-- Read 1st and 2nd sentence.

When you bring in something from another country, it is consider importing. The tax applies when your item is above a certain value for the category that your item belongs to.