Well, including laxative to judge how much people can tolerate before they get the shits isn't far off the mark from inducing depression.

FB - A little negative = meh

FB - More negative = major downer

FB - Totally negative = woah, look at the sorry state they are in now.

Cake - A little laxative = mmm this cake is special somehow

Cake - More laxative = nice cake, special, where is the roll of extra TP?

Cake - Dump it in = people home for days, the shits.

In both cases, not telling them is the ethics problem.

Basically the public trust boils down to people expecting others not to harm them. Depression can become chronic. The shits could damage someone requiring medical help.

Both are clear risks people would very likely avoid, if they knew.

FB - We want to run a depression test. Plz volunteer your feed and see if you get depressed.

Cake - We have a laxative that tastes great, but you might get the shits, plz have some cake.

See the problem?

do folks "sell cake" to see if it worsens people's depression? this is more like selling cake that secretly has nuts in it which are known to cause allergic reactions without disclosing it.

Well, it's more like if the nuts had an unknown reaction, and the idea was to figure out what the reaction was by slipping it into some cake, and then observing.

which would be massively unethical if not illegal to do as a food manufacturer