Why doesn't Zoom Player include DVD Decoder filters?
Developers of DVD Decoder filters are required to pay a nominal fee to the DVD Consortium just so they could ship these filters without breaking the law. Compound this with the per-unit license fees required by Dolby Digital and DTS just so you could decode their audio formats. This makes a free DVD player an impossibility (regardless of what you may see on some sites- usually these sites bundle pirated versions of the decoder, either hidden or outright).
Zoom Player is not a DVD Decoder. It is a smart DVD User Interface that allows you to use DVD Decoders from any vendor that supports the DirectShow DVD Interface (part of DirectX-8 and newer).
Since every display card, DVD Drive and Motherboards are shipped today with DVD Playback components, shipment of decoders was deemed a deterrent as it would just inflate the price of the product while giving little benefits.
There are also several open-source projects that play DVD content. More information.