It can only run sideways because they’re a crab, not because their legs are stiff.
Yes, the legs are “stiff” in one direction because of their anatomy but it’s like a human saying “I can’t hang by my tail because I lost it”.

I really don’t want to be a downer, but the title kind of make this a flawed message. The shark can miraculously hold a football, but the place of the crab is only as a goalie? I believe there’s always a way, but we are not naturally limited to a small set of positions.

This is basically the plot of Escape To Victory.