I remember that in videos of beta i saw TSes scoop stuff of command, yet it doesnt seem to work when i try.

Combat slaves can scoop targets, I don't think trade slaves are supposed to.

Well, in the vids i saw tradeslaves doing it, and it should make sense if they are the ones to scoop.

we didnt want to encourage tradeslaves being used in combat scenario, so tradeslaves cannot scoop only combat slaves. it wasnt the case yet at the time the vid was made.

Slaves were split in combat/trade slaves for one reason and only one. to allow slaves users to have their slaves to fight, without preventing them to use their slots for their base trades. Its bad enough people use tradeslave for aura in combat, we most certainly wont add tradeslave as scoop slave too.

i really dont get why that is so bad

It is bad enough because there is something wrong with seeing trading slaves boosting a combat squad in, let's say, Zeus. Trade slaves should not be able to do that.