OK, let's argue that. If the vendor had no knowledge of the defect at the time, how do you draw a line between "bugs" and "features"? Remember, while we're talking about something that I suspect most people would agree is a bug, how do you draw a bright line between defects that require fixing, and defects that are of the "it just doesn't work the way I think it should" variety? Some may be easy to categorize, but others...?
And what constitutes an acceptable fix? Could a vendor (e.g.) provide a patch that simply turned off this Stagefright feature? Because it could be argued that nowhere did they explicitly state the expected behavior; rather "you" assumed that it should behave in a given way.
It's tempting to want consequential liability and warranted functionality, but to be honest we've (all) been buying software for decades without it, so you'd have a really tough time trying to insist on it now on a commodity item like a phone.