Probably for shipments shipped from an Amazon warehouse (sold by Amazon, or Fulfilled-by-Amazon/FBA).
For our own labels, we print our company phone number, and that's fairly common for many ecommerce companies (although certainly not all).
It protects the customer's privacy, and in the event there's an issue with delivery, it's best the carrier contact the shipper (us) anyway.
Some folks generate their own custom labels, and may omit things like that. The data is still sent to the carrier during the "End of Day" task (or equivalent) in whatever shipping software the shipper uses.
http://static-s3.sokoloff.com/random/amazon/IMG_1880.jpg
On other labels it's after the "DF3..." ID.
Apparently they chose to leave that field off the label, but it's still sent to USPS by the software.
If you give your cell phone number out to random companies on the Internet, then yes, you can expect to get spammed. What in the world makes that seem like a good idea to so many people?
As long as the junk mailer companies pay their postage, USPS is more than happy to deliver the junk to you.
I don't have any data, but I'd wager junk mailers are a sizable portion of annual USPS revenue...