This is definitely a valid point, and I have considered making the limitation a requirement.
The jam was originally only going to have a theme (no limitation). Though I added the limitation of incorporating feline animals as a small touch of originality to the jam. I think making it optional allows for people who aren't interested in it to still participate and push their creativity further through the surprise theme. This makes the jam more open, as this is meant to be a fun game making "party" for everyone! (the original intent was not for it to be about feline animals)
However, this jam being the "Whisker Jam", it does make sense for the limitation to be required. I want to see developers incorporate felines into their games, and see how that might impact the ideas, mechanics and overall design of the entries.
It'll be optional this time around, but who knows for future jams!
Thanks for sharing your opinion, and sparking up this very interesting conversation! I'd like to see more people sharing their thoughts on this!