The reason why multiple financial institutions outside the US are rejecting US citizens is called FATCA - Foreign Account Tax Compliance Act.
It is the responsibility of the US citizen to report their assets to the IRS held in foreign accounts, not the foreign company.
There must be another reason.
Read at least wikipedia about it. You are flatly wrong. As long as we (the U.S.) have geo-political reach to impose our will on the rest of the financial world, we can impose $20 of cost on foreign entities for $1 in revenue gain. I, as a U.S. citizen who is not completely 'isolationist' financially, suffer for it but I'm not representative of the class who is being protected. Just the opposite most likely.