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If you're asking why this is done (rather than the legality of it), it's based on risk assessment. The problem with mental health issues is the lack of objective verifiability. "Depression" is different than a broken leg or arm. Insurance companies don't want to get in the business of refereeing duels between doctors arguing the nature of the mental illness.
So the limitation to 24 months is a risk management situation to keep costs down (since the total cost is going to go down, the per insured cost goes down as well).
As far as legality goes, it is legal to do so. There is a bill (H1174) that is seeking to require "parity" (basically the same amount of time) for other medical conditions, but it is not yet law.
Until there is such a law that requires parity, insurers can still discriminate between mental health issues and other medical conditions, legally.
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