My best guess is that the po-po are cutting boxes open at will and letting their dogs have a sniff at the UPS sorting facility. In case Amazon had a different opinion, I tried their chat help:
You are now connected to Anthony from Amazon.com.
Me:Hi, is there any good reason that my deliveries have been cut open when they arrive at my door? This has happened to three of my last four boxes.
Anthony:Hello, my name is Anthony. I'll be happy to help you.
Me:Hi Anthony, thanks.
Anthony:I'm sorry about the late deliveries with your order.
Me:That is not the issue.
Nothing was late.
Anthony:I'm really sorry that your order was delivered with open condition.
Me:Do you know why this is happening?
Anthony:I'm sorry; I'm unable to determine this for you.
Is UPS the shipper of the item?
I'll forward your feedback about UPS to our shipping department. I know they'll want to hear about this.
I'm really sorry about the problem you had with this shipment.
Me:Just so you know, they appear to cut open the Amazon tape but not open the box entirely.
Me:Thanks for your help. I love you guys.
Is there anything else I can do for you today?
Me:What, no "I love you" back?
Anthony:We look forward to seeing you again soon. To close this window, please click the "end chat" link.
Me: That's cold, dude.
Anthony from Amazon.com has left the conversation.