No reason other then the "Worlds Leaders" using a flimsy pretext to invade a sovereign state. Which is a fundamental no-no of the UN. I have no love for any "leader" abusing their position.
If this is what was agreed to in the UNSC then it would at least be seen to be and promoted as "legal".
The extensions and mission creep are opening up all sorts of future attacks which are now "accepted" by world leaders. Bin Laden's murder for example.
It may be us on the receiving end one day and the "justification" for all these precedents will be "recalled".
It is making the whole world less save for "civilised" populations.