They even used animals for experiments.
That's not even partially true, since the space dog was split accidentally to begin with. Its halves would have died, just like Kirk's were going to, if they hadn't tried rejoining it. The procedure would have been the same, even if it had been done after Kirk was rejoined, and the animal likely still would have died.
The charge of using animals for experiments would stick if they'd taken an initially healthy animal and split it on purpose, just to see if they could rejoin it.