That is my point. A great deal of fentanyl does cross our borders illegally, and leads to a great many American deaths -- BUT it is not coming with immigrants, so arresting or harassing immigrants does nothing to stop the drug-overdose deaths. Arguably, the money our government is spending to punish immigrants is being diverted from efforts to keep fentanyl from crossing the border, and may result in more deaths (one hopes not, but I guess we'll find out).
My point about behavioral health is that it's the practice of figuring out what steps change people's behavior in healthier directions, and what steps produce less death and suffering. Conservatives typically have a hard time with that mode of thought, because it requires us to stop saying and planning, "Drug users are bad people. Sweep the places where they do drugs, arrest and punish them." Instead, you have to find interventions that actually work, like needle exchanges.