You can fight anytime in the Fighting Matsuri time frame, just need to win 6 battle, so asking someone to sign-up for you is totally possible
Sagewar Battlefield requires sign-up so they can distribute players into 3 faction before-hand in a reasonable fashion, otherwise there can be a faction with most of top players and would bring no competition
Fighting Matsuri requires sign-up so they:
> Can have zombie teams. Not sure if you read it right but if the matching take too long then you will fight zombie teams and that would be impossible without pre-registrigation
> Make it rather fair (?). Those 3 hours have several events that may boost up your fighting power, so to make it fair they required people to sign-up at the same time