Labelling the garage delivery as only to Amazon's benefit is a bit disingenuous. Package theft is a pretty bad issue in many places so having deliveries dropped behind a locked door is also a benefit to the user.

As for your security concern it's not unfounded but if your garage is built like most in the US there's probably already a locking exterior grade door between it and the outside because a garage door isn't that great as a security barrier to begin with unless you remove the pull cord that unlocks the door from the carrier.