Because people have no clue wtf they're talking about.
Notice the two comments here:
1st one doesn't reference what spell it is, so obviously not a necromancer.
2nd one says they have cooldowns and high mana costs(which is the whole point of necro) then states that they're messed up.
Anyway, if we assume the first comment is about firebolt, then it has 2 features that make it rather unusual for a necromancy spell.
1. It is fully resistable. It is the only necromancy spell(iirc) that is fully resistable.
2. It's mana costs used to be split among 2 stages as was its damage, due to coding restrictions, now it's only one, so it's still getting minor tweaks to put mana usage right for the damage.
I actually just did some modifications on it to put it more in-line with what it was on t3a, lowered damage a bit.
Now as for the whole "why enable shit that's abusable".
In 2 words, it's not.
In full sentences, it's not really abusable, at best anything that goes in is mildly unbalanced ignoring any bugs that people find that can be abused. If something gets put in, i'm reasonably certain the basic numbers are fine and it just needs playtesting for final tweaks.
Firebolt needed tweaks because it needed full testing given resist/parrying since it's affected by them.