AaronOfBarbaria
Adventurer
The rules apply only when the DM feels they are appropriate, actually - so a typical peasant will or won't notice an invisible orc in the darkness as determined by the DM.The rules apply equally to everyone, though. A typical peasant will still notice an invisible orc in the darkness, unless it takes an action to hide. Or that's what the rules say, at least. The only difference between an adventurer and anyone else, as far as these particular rules are concerned, is that adventurers tend to have a better chance of detecting an orc who is actively hiding.
Yes, the DM can choose, as you appear to do, to have the rules apply equally to everyone - but that is not a requirement made by the game. It's not even a suggestion made by the game.