I think this is the real reason why it was cancelled.
I'm don't think anyone would have been throwing things at him (that's assault), but you can be sure the parade would have been attended by many veterans and other people outraged by the hero's welcome for a deserter.
And they likely would have expressed their displeasure in various ways protected by the First Amendment.
BTW, does anyone know the status of the gal that tossed a shoe at Hillary a while back. Is she still in custody or what?