Openness and honesty are two essential characteristics of the relationship between investigator and research participants, but these are repudiated when participants are subjected to research the means or motives of which have not been honestly explained.
In one study, male undergraduates were falsely led to believe they had been sexually aroused by photographs of men and their responses to such information studied. In another, subjects were surreptitiously given LSD.
Deceptive social science research is an interference in the subject's freedom of informed choice. It also makes a mockery of the trust one human being places in another, especially someone in authority who would normally be assumed to be trustworthy.
Since most experiments are utterly harmless, it is a waste of time and energy to impose upon them the kind of requirements full explanations might entail.