We’re sorry that you were unhappy with the inscription on your in-laws’ marker.
The monument company doesn’t determine what goes on the marker. That information is provided to them by whoever purchases the marker. From the way you described the situation, it sounds like the parents are the ones who decided who would be listed. Perhaps you could talk to your wife about it to see what her parents had in mind.
As for how common the practice of omitting certain people is, it’s hard to say. Each family has its customs and ways of doing things. If you take a walk through a cemetery you’ll see a wide range of designs, from simple names and dates to more complicated wording such as what you have listed.
In any case, we can understand why you might be peeved about the omission, but it’s not the monument company that is to blame.