These mochi balls are shelf-stable and designed for decoration more than consumption. I bought one of these a few years before I left Japan because it had an adorable little plastic tiger on top of it. I had it for about a month and then decided to pop it open and sample the mochi. I didn't expect it to be like freshly-made mochi. What I expected was something akin to the dried stuff that you buy in bags in supermarkets. It was far "worse" than that shelf-stable mochi. It was so tough, even with heat and moisture added in, that I felt it was inedible.
While I may have gotten a bad batch, my guess is that these are "mochi" only in name and that these are really meant only as decoration. I'm guessing Japanese people already know this and wouldn't even attempt to eat them when the holiday is over - much as we might view fruitcake. ;-)