Spamming any move without thinking is the easiest way to get destroyed by a player who knows what they're doing.
Fireballs let you control the distance between you and your opponent and restrict some of their options. If you throw one out without thinking it can easily be jumped over, negated with an invincible attack, etc, leading into big damage against you so it isn't like it's cheap and unwinnable against. It's a legitimate part of the game, even if it can be annoying to play against.
Complaining that the guy threw out too many fireballs would kind of be like him complaining you hit him too many times and that's the only reason you won.