Frisk was given to Grillby after major consideration. You would be right if you said the officer in charge of Frisk didn't like monsters, but Frisk did like the monsters, and some never tried to harm them, and they had to go somewhere.
The officer wasn't going to send a five year old back to the orphanage they ran away from, though they did start filing paperwork to investigate the orphanage as Frisk was never reported missing.
"You want to send Frisk to one of them!" The officer yelled. "Frisk was covered from head to toe in bruises and blood." "Not every monster attacked them, some even helped." The officer looked down, "I guess, it's just their so young." "They are which certain monsters took advantage of. This lead to them feeling the need to advocate for the monsters, this may make them feel better." The social worker continued, "Besides Frisk was almost certainly abused in the orphanage, this way we can keep an eye on things." The social worker placed her hand on the officer's shoulder, "And they can feel needed."