I personally would not want this. Although it is in a way logical, as you say, that the actions cannot have a later deadline than the project as a whole, most tasks will not have a specific deadline at all, and those that do will have it on an earlier day than the project. So, essentially, this feature would add a lot of clutter, and would also make it virtually impossible to see which of my tasks have a real deadline of their own.

What I would suggest instead, perhaps, although I do not need it myself, is if Doit could somehow show two deadline dates, both a normal task deadline AND an (automatically) "inferred deadline". This would enable you to see clearly whether the individual task itself has a deadline or not (usually not, probably), and would still allow you to see the overall deadline for the project.