Why does it seem unfair?  No one forced them to get married, and no one is forcing them not to enter into a "disregard community property" agreement.  (It doesn't have to be premarital.)  Generally, IRS doesn't care as long as someone pays tax on all the income.

A page of history is worth a volume of logic.  Oliver Wendell Holmes Jr. said that.  Do you know why there are joint returns?  Because in the early days of the income tax, married taxpayers in community-property states avoided the higher brackets by splitting their income, following state law.  And Congress didn't include "community property laws shall be disregarded" to the statutes, as it does in some cases now.  What it did was to make it possible for everyone to use lower brackets, by filing joint returns.  The top of the form may say "Individual Income Tax," but that's just another lie, we all know it's a tax on couples, enacted back when everybody was expected to marry and nobody was expected to divorce.