The basic idea is that when you use a bondsman, you are entering into a contract where you agree that you will show up so that the bondsman can get his money back. The contract also says that the bondsman can chase you and arrest you by force if you break the contract. So, because bounty hunters are not law enforcement officers investigating a crime, just private individuals enforcing a clause in their contract, they can do all sorts of nasty tricks, follow you accross state lines, break into houses without a warrant, and so forth.