It was a mutual option. Had the Phillies exercised it, Morton still could have voided his end of it, choosing total free agency over the security of a one-year deal (which did pay more than the Astros gave him, albeit for two years).
I suppose it is possible the Phillies simply decided, especially given the 40 situation and the return of Hellickson, that they did not want Morton back, period. But one would think there was at least a conversation, in which the Phillies asked his agent what kind of offer they'd accept if the option was torn up, and/or whether they'd exercise their end if the Phillies did.
I'm not saying Asher should have been a firm answer in the off-season (any more than they were counting on Eflin, Thompson and Nola to be fully healthy) but in retrospect he was probably worth a 40-man spot, even as limited as they were (you would have had to part with Luis Garcia, keep Stassi in AAA or go with Valentin over Nava. Given they now have Ty Kelly and Valentin got hurt the latter scenario sounds pretty good, though Nava's been useful).