Flatly untrue.
Romney's numbers referred to those who don't pay one of the many taxes, federal income tax.
This group continues to pay many other taxes, from SS to sales tax to other local and state taxes. Comparisons between US and European taxation are frequently flawed by comparing US federal taxes only, ignoring the large state and local burdens, which are generally much higher here.
Not to mention a huge amount in indirect taxes, although they are of course generally unaware of it.
I don't know where you are, but here State and local taxes, from income, sales, and property taxes combined are a fraction of the Federal Income Tax I pay (<20%).
That does not address the Federal excise taxes, cell phone and other Federal telecommunications taxes/fees, and other hidden taxes.
As an aside, being disqualified from voting in a Federal Election would not necessarily preclude the States or Localities making their own rules about who can vote in State and Local elections, respectively.