The answer is that it isn't cruel if it is done right by observant and responsible farmers who attend to the various needs of these wonderful creatures.
Pigs need to be fed regularly as they can get sick if they start to starve, and they must also be cleaned and groomed regularly to ensure that they don't catch anything. Responsible pig owners in a farm need to make sure their pig is always happy, healthy and content. Remember they are intelligent animals and as such require the proper stimulation in order to live happy and well-rounded lives. As well as this, pigs can get injured, so they must be treated quickly in the case that they are hurt. This is where the supervision comes in to play.
Pigs can even become aggressive at times, which means that they may need to be kept in a separate pen from the rest of your animals. This doesn't mean that there's something necessarily wrong with the health of the pig, but as complex animals they are all different and carry with them different personalities.
A pig’s diet should be varied, so that it gets all the nutrients and vitamins it needs to grow. Not providing them with a carefully selected diet is cruel as they can pretty easily become malnourished.