How does LE know that the bad guys are not in your house? What if the bad guy was standing around the corner holding a weapon on your loved one to get you to say the right thing?
They don't but it is impossible to prove a negative. It is not my job to prove there is not a bag guy in my house; it is the role of law enforcement to show probable cause.
The only way to make sure there are no meth labs in a given geographic are is to search and meth is very harmful to society. Fugitive searches could use the same justification. It is a very loose standard.