Objects may be dropped from an aircraft as long as adequate precautions are made to avoid injury to personnel or damage to property on the ground.
Although regulations allow for the dropping of objects, this exclusion is not intended for mischief or hazardous flight conditions to occur. Remember, the pilot in command may not operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.
14 CFR 91.13 Careless or reckless operations
14 CFR 91.15 Dropping objects