According to ABC News, here's another reason Mitt Romney might have been reluctant to release his tax returns, and even now seems unwilling to release returns from years past:
In addition to paying the lower tax rate on his investment income, Romney has as much as $8 million invested in at least 12 funds listed on a Cayman Islands registry. Another investment, which Romney reports as being worth between $5 million and $25 million, shows up on securities records as having been domiciled in the Caymans.
The Romney campaign insists that Romney pays all U.S. taxes on all his income. But there may be other monetary benefits to setting up in the Caymans, according to people who understand this type of thing:
Tax experts agree that Romney remains subject to American taxes. But they say the offshore accounts have provided him — and Bain — with other potential financial benefits, such as higher management fees and greater foreign interest, all at the expense of the U.S. Treasury. Rebecca J. Wilkins, a tax policy expert with Citizens for Tax Justice, said the federal government loses an estimated $100 billion a year because of tax havens.
Maybe there's a nuanced interpretation here that leaves Romney completely faultless in every way, but, unfortunately for Romney, most Americans don't have a nuanced understanding of offshore taxation schemes.