They do support them, but for the customers who received them pre-merger. I have three of them, and actualy just had one replaced. I am on a legacy TWC video / internet plan. If you've upgraded your plan, or are a new customer post-merger, they are not available to you.
As to the main reason why they are not supported going forward:
A DTA is a one-way device. There is no way for Spectrum to read signal levels remotely. This limits the ability to troubleshoot over the phone. Spectrum can read signal levels remotely via the set-top box, not the DTA.
Two other points:
1) Only the first DTA unit per subscriber connection is free, and only for the first 12 months of service. After one year there is a monthly rental charge of about $5. Spectrum will begin billing you starting from the first day for additional DTAs needed for any other viewing devices in your home. But again, to qualify for the first year free you were required to have the DTA on line BEFORE your area converted to full digitally encrypted service.
2) Each DTA MUST be rented from Spectrum. It is not something you could purchase from an electronics store or dealer and install yourself. "Cable-ready" TV sets will no longer directly connect to the cable system.
The net effect is that you would get free basic service to one viewing screen for twelve months, during which you must decide how you wish to proceed with your video viewing options. After that you will have to pay for each viewing unit, and you can have a mix of DVRs, full featured set-top converters, and the econo-DTAs for a limited time.
Also from a monetary perspective...since the DTAs are one-way only they do not provide access to On Demand, meaning the possibility of a customer ordering a movie that brings in extra $ is lost.