Not noted in the other, excellent answers:
Inexpensive bikes have a tendency to not be well cared for - they're purchased by people who either need inexpensive transportation or for kids (who are notorious, as a group, for not taking good care of expensive things*). More expensive bikes tend to be better cared for by riders who actually care about the bike they've purchased and would prefer not to have to replace unnecessarily damaged components.
When a casual rider of an inexpensive bike stops riding, he can often be seen simply dropping the bike to the ground and walking away*. If the bike happens to drop on the right side, the rear derailleur will hit the ground. Even from the relatively low height of a rear derailleur, damage can ensue. The derailleur protector can take significantly more hits than the derailleur itself can before damage build-up is enough to cause riding issues.
More serious riders of more expensive bikes rarely "drop" their bikes when they're done riding. The bike is carefully balanced against a wall/tree/rock/etc to ensure that the precious componentry and paint job are not damaged. If the bike is laid down flat, it will be laid on its left side precisely to avoid having either the front or rear derailleur come in contact with the ground.
*These statements based upon personal observation of my own children not heeding parental instructions to not drop the bike when finished riding, and personal observation of other people treating their BSOs this way. No offense intended to any casual rider who may happen to actually care for her bike.