What is the harm in letting them go?

The question that occurs in my mind is that why U.S. government is arresting and detaining those migrants going back to Mexico for the interviews of consular. These migrants are going to apply for waivers of inadmissibility because of citizen family members.

The U.S. government is leaving a stone unturned in arresting those seeking for asylum as they move to Canada. The point is that if in U.S., the seekers’ asylum appeal will keep pending, and the seekers cannot sit and wait there. If the claim of the seekers stands rejected in U.S., there is no point that seekers would wait to get deported and risk their life to the fate. The asylum seekers have all the right to move to Canada if they want to claim for the asylum there.

I am dumbstruck with this step of U.S. government of arresting, detaining and deporting those unauthorized migrants who are themselves leaving the country in a peaceful manner. These steps hold no rational ground and I cannot find any explanation as such why the government is meting out such a behavior. The objective of the government on the policy lines is also not making any appeal to me. Immigration rights must be taken into consideration before taking such steps against the immigrants.