If it didn't punish you for not using all your moves it wouldn't make almost any difference. Knocking the player down for attempting to abuse what has been placed as an invalid tactic is perfectly legitimate game balance. The move still has some damage and knockback, and still has the same hitboxes/speed, so it's not like it's entirely useless, just you'll be a better player if you can incorporate all of your moves well rather than just relying on 1-3.
It's been deemed an invalid tactic by whom and since when? Since when has
any fighting game community deemed spamming the same few moves, especially for shield pressure, guessing game and openings for combos an invalid tactic?
In fact, it's the
norm for the majority of fighting game out there. Most moves a character has are useless unless used as Wake-Up (of which Smash has limited use), guessing-game one-hit pokes (of which we have limited use as well as we a different shield game) or simply as a follow up from a launcher/jab.
I've played Soul Calibur, Street Fighter III: 3rd Strike and Naruto: Gekitou Ninja Taisen 4 (not that deep, but still) on a high to semi-high level, and seen high level play of Melty Blood, other Street Fighter games, the Guilty Gear XX series and more and they're
all about using a limited amount of moves for openings/launchers/mindgames/Oki-zeme. It's because most moves are
useless as standalones because if blocked, they lag too much to either continue pressuring or they lag so much you'll be
punished just for using them.
Out of 24 or so moves, Chun Li regularly uses 4-5 in Street Fighter III: 3rd Strike... on the
Pro level.
While incessantly spamming the same move 90% of the time you use a move is bad, this is extreme (Zelda used the fair, like, 8 times).