The problem here is that in all of these situations the characters were of sound mind, and able to make a logical decision regarding whether or not they wanted to continue living. It is very clear that this was not the case with Sasdren, he was very clearly severely depressed, and was very obviously not of sound mind.
Exactly. This is the critical point that keeps being ignored. He was not competent to make an informed, rational decision.
And the other critical point that keeps being ignored is that cultures are not the same things as individuals. So what if S'ti'ach culture did have some kind of "suicide is okay" belief? It's nothing more than racial profiling to assume that any given individual S'ti'ach must share the general belief of his culture. For all Crusher knew, when Sasdren was in his right mind, he would've been vehemently opposed to his culture's (hypothetical) acceptance of suicide. This is why you simply DO NOT allow someone to die based on any kind of assumptions or speculations or guesses, let alone racial stereotypes -- you only do it if that SPECIFIC individual has a DNR on file. You default to saving the patient. You never default to letting them die. That should be screamingly obvious.