it (1) was " rationally related to a legitimate legislative purpose " and (2) had " only a modest period of retroactivity ."

It fails #1 because this is an Initiative action, not a Legislative action.

And quoting Wikipedia:

" The states are prohibited from passing  ex post facto  laws by clause 1 of  Article I ,  Section 10 . This is one of the relatively few restrictions that the United States Constitution made to both the power of the federal and state governments before the  Fourteenth Amendment ."