My thoughts, as posted on FB:
"The reason, as explained to me, is that the Ta"Z is the only one who holds that Ma'ariv effects a change in the status of the day. However, if I'm not mistaken, since it is a Da'as Yachid that affects a DeOraisa, we go LeChumra. Sephardim have an easy way out, as the Ta"Z is not cited as normative Halachah for them in other areas like he is for us."