I was wondering, what's the logic behind human Cultists taking reduced damage from plasma and laser?
There is no logic behind it. Most of the things I make come straight out of my personal ruleset so my mods are "balanced" around that. I've changed my ruleset so the weapon classes (Beginning, Laser, Plasma) have different roles instead of being almost straight upgrades and I've buffed most of the aliens in some respect as well (Snakemen resist lasers and are tougher, Mutons are walking tanks (3x vanilla armour) but aren't so good in the psi department, Ethereals are weak but have good reactions and psi strength >100 etc.).
Cultists are fairly weak and, if you can cope with the psi attacks, they don't pose much of a threat. Adding a slight resistance to lasers and plasma weapons means that plasma weapons are viable but will work about as well as autocannons, ensuring that the missions are still decently difficult when you have the psi aspect dealt with.
Alternatively, their resistance to lasers and plasma weapons is the result of alien hazing rituals. Who knows?