Politicians write these rules - they are legislators. Here is a a history of the tax inversion.
"About 51 U.S. companies have reincorporated in low-tax countries since 1982, including 20 since 2012. A lot of drug companies are doing it, and low-tax Ireland is a popular corporate home. They’re doing it despite a 2004 law that legislators had promised would end the practice, despite rule-tightening by the Obama administration to limit it, and despite two decades of efforts by the Internal Revenue Service to rein it in.
A change of address doesn’t necessarily mean a real move. Companies are free to keep their top executives in the U.S., and most of them do.
Most importantly, perhaps, companies that invert overseas can take advantage of the generous U.S. system of interest deductions for payments to their own affiliates abroad — benefits that are only available with a foreign parent company."