She was in the Cameron Cabinet that set the rules for the Brexit referendum therefore she shares collective responsibility.
In any event she, as the leader of the Conservative Party, might reasonably be asked to account for apparently acting hypocritically in setting one margin in referendum (a) and another in referendum (b).
It was repeatedly pointed out that the winning margin in the EU referendum would have been insufficient to legally support a strike on the tube.
Not all rules for referenda are set the same……its upto the government of the day to set the rules, irrespective of any previous referendums