Mandatory or Forbidden, Banned or Endorsed

 Peasants have very simple (for lack of a better word) minds. Remember very few can look beyond the first result of a google search. Not [won't] but [can't]. You might think, "they're right there, how can they not see them," but that's because you're not a peasant.

 A peasant can't even fathom social reality, they have no resources at all to deal with, like, physical reality.

 Consequently, if the lord doesn't criminalize suicide, the peasants think he's endorsing it. They believe they should ram a knife in their own eye as soon as they work up the courage to do so.

 You might think, "shouldn't the dying part of suicide be enforcement enough? Think about who would want to break this law," but of course peasants don't really have desires per se. Their minds aren't sophisticated enough to have identifiable or distinguishable values.

 The only thing a peasant can truly be said to want is to obey. The peasant believes everything not specifically forbidden is mandatory. The idea of 'optional' is too rich for their blood. It would require parsing an if-then statement, which is beyond their mental capacity.

 You can still highly limit suicide by making the peasants unaware that suicide exists. Peasants won't come up with the idea on their own, and thus won't think it's mandatory. Journalists have something to say about that, though, if you live in a country that has decriminalized/legitmized journalism. 

 Humanity is basically evil. A grass monkey won't do anything useful unless he's forced to. Peasants grew up/evolved in an environment where coercion was taken for granted. Consequently the peasant is very poorly adapted to environments where he's not fully enslaved to his family. (Which is his whole society.)