People are assuming too much things about the modus operandi of the FMS here. I would at this time advise that people check out an operating manual of the Airbus in question, before assuming it takes account for FF and the how's and do's, and even, accuracy of.
Pilots should know better to always check the performance tables. Always, no workaround here, always is the way to go. So, these men didn't do it, maybe the focus should be on the reasons why they didn't, because i am pretty sure that they we taught like that, some time or another along their training.
FMS is a tool that supports pilots in conducting flights, not a tool where pilots supports their flight planning altogether. Proeficient pilots crosscheck the FMS from performance charts, take conclusions about the results and act accordingly. No more to it. Pilots that trust whatever the FMS throws at them are in a collision course with a problem.