@brody In most cases I've seen, they don't actually require sign-in and work without authentication.
My guess as to why they do this:
1) It's relatively easy data to scrape
2) It allows you to provide more relevant data to the customer (i.e. what's in the packages)
3) It allows them to obfuscate certain elements. I.e., if you look at actual tracking, you'll see the label created a day before it was actually shipped, but on the vendor website, they combine those to make it look like it went out immediately.
4) What company doesn't like to have their branding slapped all over every customer interaction?