My Lion King Forum

Was it because:

1.) It was retaliation for all of the throw rug jokes.
2.) Because he feared Zazu might recall being hit by him right before Mufasa's death.
3.) So Zazu couldn't leave (Zazu had made some comment about leaving if Simba, on his current course, became king. He might have actually left when Scar became king and brought in the hyenas, and perhaps Scar didn't want him to leave.)
4.) Merely so that Pumbaa could rescue him later.

I think Scar imprisoned Zazu for his amusement as he has grown bored with his responsibilities as king.

There's a possibility Zazu might recall being hit by him, so Scar may be driven by that motivation among others in addition to imprisoning Zazu for his amusement like TheLionPrince mentioned. I kinda wonder if he put Zazu there right after announcing Mufasa's death to the lionesses or later on during his reign. The It's a Small World part still makes me lol.