Not a bug, worked like that before. Hammer skill for zon is different case, special mechanics.
But for almost everything else, spell damage always affects direct melee hits, and amplifies whatever elemental damage you have (that includes poison, and all elemental passives, sin ways, nephalem spirit for barb, flat ele damage added to attack skills, basically all sources).
It isn't very useful however because it will only carry with direct melee hit to 1 target. So for instance if you use whatever skill you are using, that multihits like crazy, for example stormblast. You gonna hit that 1 monster that you originally aim with 100% WDM from normal weapon swing ONCE per animation (and that gets amplified by spelldmg) and then 30% wdm stormblast will also go through that monster (and through other targets if they are on its path), will multihit it bunch of times (due to its mechanics) and none of those hits gonna get multiplied by spelldmg. But usually its gonna do way more damage anyway, because of how many times it hits.
In case of poison and stacking duration via various mechanics, there actually is benefit of building spelldmg for it, because multihitting doesn't matter with poison. And windform happens to count as direct melee hit (with 150% wdm and extra poison). This is quite unusual case here, but it works, you can reach respectable amounts of damage. Just only vs single target, this has zero application for actual farming/clearing, your procs will deal just whatever listed tooltip of snake stance+windform (+whatever passives you have such as amatoxin oskill for example) and not include spell bonus.
I can give one more super ancient example when ppl used this.
Very long time ago, ppl used to kill bosses as sin, with normal attack skill (shadow refugee which hits 1 target with 1/1 wdm). Reason for that was to avoid aoe that for example killed statues in astrogha fight to heal it. Another reason was, it was worth to have multi-elemental attack, because of way how bosses were designed, they dropped their immunities randomly so with all elements carried by your hits, you increased the odds of damaging the boss. Vs situation if you had to get it to drop specific immunity, if all your damage was 1 element type, focused on one skill.
You maxed all ways, and then stacked spell damage, so your direct melee hits were dealing ways*spelldmg. It wasn't super good, but worked well enough to get nice clean kills.
It is no longer useful, with power creep, various new skills added to the game, buffed, but there was time when it was actually a thing to get special gear set for those "0 aoe" boss killer builds.