Why do many of these straight trucks have trailer brakes?

Nope. That builder's label says one steer axle, one intermediate axle, and one rear axle. The other two positions on the form are empty, thus there are no other intermediate axles. Freightliner used the same label for anything with 2 axles up to 5 axles. If there's nothing printed next to "Second Intermediate Axle" then it means the truck to which it is affixed has no second intermediate axle. Ditto with "Third Intermediate Axle". Since there's nothing printed there, it doesn't have one.

The "First Intermediate Axle" and the "Rear Axle" are a tandem. Each axle is rated for 20,000 individually. But that does not mean you can run 40,000 combined on them if they are closer than 10' apart.