I remember a response/comment from FDA (not sure where, now, after how many hours of docs, threads, and webcasts), where they justified the 2007 grandfathering date for "ENDS" (still hate the term, still intend to use quotation marks and air-quotes for it indefinitely) because one of the things they were looking for during the comment phase, and did not receive, was sufficient grounds that they could choose a different date for these newly deemed products.
I understood their position to be that, once they were deemed tobacco products, then that's what they were, and always had been. In turn, all the products they included in the deeming became "tobacco products" that had just gone unchecked since their introduction to the market- they were all "tobacco products" last year, and the year before, just no one knew it yet.
Since the FSPTCA grandfathered "tobacco products" prior to 2007, and anything they so deemed was, also, now a "tobacco product", trying to assign a different grandfather date would have put them in a position where they were applying the law unequally to products in the same category.
Specifying a different grandfather date for vaping would have opened them up to legal action from BT, and I bet they'd rather face our lawyers than theirs- especially since most of the plaintiffs would eventually be working for the FDA.