Your working AND non-working dehumidifiers (and A/C unit, mini-refrigerator, wine cooler, etc.) typically contain a refrigerants such as Freon. Freon as defined as a class of liquid or gaseous fluorocarbon or chlorofluorocarbon products, used chiefly as refrigerants. This is why dehumidifiers and other devices containing Freon/refrigerants are not collected with other electronic devices.

Here's the environmental spiel provided by the EPA Stratospheric Protection section. EPA regulations (40 CFR Part 82, Subpart F) under Section 608 of the Clean Air Act include requirements for the safe disposal of refrigeration and air-conditioning equipment. These requirements are designed to minimize refrigerant emissions when these appliances are disposed.
Single Dehumidifier
The good news is the majority of our local scrap yards can provide safe disposal of items containing refrigerant. A list of local scrap yards can be found under our  "Where do I recycle this?" directory.
EPA - Safe Disposal Brochure