Because those legends balance other pokemon (sometimes, they are the only counters to certain pokemon), and numerous competitive players use them on their teams and won't bother training a new pokemon just for this league?
The difference, then, is why ban something that doesn't need to be banned?
Okay, I see the swift swim thing, but the main point is about certain "legendaries" like the useful Virizion, or hell, like Articuno, which is simply not competitively viable. Or the Latii's. They're the only truly viable checks to a few other dragon pokemon IIRC, but not so overpowered any more (hell, my latios gets KO'd most of the time I use it), as standard wifi rules prevent Soul Dew, plus it's not available (other than through hacking, obviously). Given the strength of various new pokemon, Latios doesn't last too long, and latias is outshone in many aspects.
The difference, then, is why ban something that doesn't need to be banned?
Okay, I see the swift swim thing, but the main point is about certain "legendaries" like the useful Virizion, or hell, like Articuno, which is simply not competitively viable. Or the Latii's. They're the only truly viable checks to a few other dragon pokemon IIRC, but not so overpowered any more (hell, my latios gets KO'd most of the time I use it), as standard wifi rules prevent Soul Dew, plus it's not available (other than through hacking, obviously). Given the strength of various new pokemon, Latios doesn't last too long, and latias is outshone in many aspects.