Basically, railroads today can't call a "caboose" a "caboose" without the car in question equipped to comply with FRA and, in California, PUC regulations for cabooses. Compliance with these regulations are costly and, for the purpose of most cabooses left in existence today which are simply used as a riding platform for crews in long shoves (reverse moves), all of these costly appliances and pieces of equipment are unnecessary. So in order to "re-classify" the car to evade FRA and PUC caboose regulations, they weld or lock the doors to the car's interior and call them "shoving platforms". That way they can serve their basic, contemporary purpose and the RR doesn't have to spend a bunch of $$ to make the car compliant with actual caboose regulations.

It may LOOK like a caboose, but if you see one out there these days chances are it's a shoving platform.