Because the ambiguity is the most crucial bit.
People need to understand this. If you have ambiguity around a supposedly clear line drawn by the law, then you get a penumbra of extra territory that people will steer clear of. Then, when you're talking about responsible therapists, they're going to make extra distance between their counselling and the penumbra. What trans activists are after is to make definitions of Conversion Therapy sufficiently unclear that responsible therapists are going to feel uncomfortable with providing any helpful exploratory non-affirmation therapy.
You don't need to find individual therapists in breach of the law to scare others off. It's akin to the "chilling effect", although this time for therapy rather than for free speech. But when this is happening those who advocated for a broad vague Conversion Therapy law will be saying "see, no therapists are falling foul of the law, all of those concerns were groundless".