According to a 2011 Gallup poll, 80% of Americans think suicide is wrong.
Dear Americans: why is suicide wrong? Presumably, the nature of the "wrongness" of suicide, whatever it is, justifies the de facto suicide prohibition.
I suspect that a large part of that 80% would identify a religious reason for the moral wrongness of suicide (not that I think folks have much introspective access to their true motivations). However, only 51% think abortion is morally wrong, and 23% think divorce is morally wrong, despite strong religious condemnation of both practices.
Even more than religious convictions, I suspect that the perception of suicide as wrong is, at heart, about the suffering of people "left behind" when we kill ourselves. The suffering of those left behind is especially salient. But even if suicide does major harm to those left behind, to our mothers and sisters and best friends and teachers and lovers, this does not justify a suicide prohibition. A prohibition enacted for such a purpose is identical, economically, to a regressive tax.
Regressive taxation is when poor people pay a higher share of their income in taxes than rich people do. It characterizes our current economic system in the United States, but it is generally regarded as undesirable to take from the poor and give to the rich.
A person who genuinely wishes to commit suicide has, by her own measure (the only one that counts), negative expected future utility. She expects to suffer in the future more than any future benefits are worth. The suicide prohibition prevents her from improving her position (from negative expected utility to zero expected utility). It does so for the benefit of those who do not wish to commit suicide - those with positive expected future utility.
The suicide prohibition, when justified on the basis of harm to others, punishes the least well off for the benefit of those much more well off.
It takes choices from the poor to give pleasure to the rich - without consent and without compensating the poor in any way.
Another word for that is exploitation.