I know, I know. I soak EVERYTHING. I even soak counters and floors! But plastic is a different story. I hate washing plastic. Despise it. No matter how much soap I use, or how much I scrub, once I rinse it, there always seems to be a greasy film, and I end up having to wash it again. This is a pain, and the more greasy surface area there is to scrub, the more of a pain it is. Soaking plastic in a sink full of dirty dishes ensures that every square millimeter of plastic is covered in dirty dish slime. It's a lot easier to just focus your scrubbing, rinsing, and rewashing efforts on the comparatively small section on the inside of the container that is actually dirty.
If the inside of the container is crusty or gross, you can still soak the inside by filling the container with water in the sink, but not putting the plug in, so the outside doesn't get tainted.