Sticker on caption phone says that using the captions can be illegal. Why?

Question

I was testing donated electronics at a local non-profit, to decide what to put on the shelves and what to recycle, when I found this:

Federal law prohibits anyone but registered users with hearing loss from using this device with Captions On.

I can understand the loudness warning on the ear end of the handset, but I'm puzzled as to why captioning a phone call would be illegal if you didn't need it.

What were the circumstances under which that prohibition came about? What is it trying to prevent? For example, is the prohibition based on concerns around eavesdropping or wiretapping? Is the captioning technology licensed under a restrictive license that permits its use only as a disability accommodation? Is the purchase cost of the device subsidized under some disability benefit program?

I've seen commercials for captioned phones, that are clearly not targeting me, but as far as I can tell, none of them mentioned that it's actually illegal for me to use their product for its advertised function.

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Per the comments, here's the bottom too:

Answer 1

This is the Fed (FCC) saying "I'm paying for this, so I get to determine who uses it."

The service costs money. It is not automagic speech to text, but rather there is often (usually?) an actual human typing it in. Carriers front the cost, and then get compensation from the Fed. The Federal govt picks up that cost. They don't want people that don't need it due to hearing loss making use of that service and its associated costs.

From the FCC: Title IV of the Americans with Disabilities Act

(b) Availability of telecommunications relay services: the Commission shall ensure that interstate and intrastate telecommunications relay services are available, to the extent possible and in the most efficient manner, to hearing-impaired and speech-impaired individuals in the United States.

The Carrier, for compensation from the Fed, has to provide logs of the phone number associated with the service. Each Carrier has their own certification procedure to ensure that the person has a documented need for this service.

https://www.ecfr.gov/current/title-47/chapter-I/subchapter-B/part-64/subpart-F

47 CFR § 64.604(c)(5)(iii)(D)(2)(v)-(vi)

(v) Incoming telephone number and IP address (if call originates with an IP-based device) at the time of the call;

(vi) Outbound telephone number (if call terminates to a telephone) and IP address (if call terminates to an IP-based device) at the time of call;