Theft Protection is a product offered by car rental companies which protects car hirers from the risk of their hire car being stolen. It is separate and different to Collision Damage Waiver which waives damage to the hire car whilst you are hiring it. Often however they do exclude the first part of the replacement costs of a vehicle and this 'excess' or, as it's called in the US, 'non-deductable' can be insured.
Theft protection excesses can be higher than collision damage waiver excesses so, if taking an excess protection product, ensure it covers you up to the higher of the two.
The options on excess insurance are either directly at the car hire desk - all car hire firms offer policies to reduce or remove the excess, but this is almost always very expensive, and can cost more than the car rental costs itself (a budget figure would be between £15 and £20 a day for Collision Damage Waiver and Theft Protection excess cover) - or via a third party 'stand alone' insurer such as those offered through the MoneyMaxim car hire insurance comparison service when more comprehensive cover costs from around £2.00 a day or under £35 for an annual policy (covering both the theft and collision elements).
The third party policies are more comprehensive as they also cover parts of the car that the car rental firm policies exclude, such as the wheels, roof, underbody, tyres and glass (normally both windscreen and mirrors). A burst tyre or chipped windscreen can result in a sizeable bill otherwise.
In the US and Canada residents are able to cover their rental car on their normal car policy. On cars either booked directly with an American car rental website or directly with an American car hire desk, visitors from Europe would need to buy insurance separately to cover the risk of the hire car being stolen. Again these policies offering full Collision Damage Waiver, Theft Protection and Supplementary Liability Insurance are much cheaper through the MoneyMaxim car hire insurance comparison service than they would be bought directly from a car rental firm.