Good question! It also brings up the importance and historical significance of a location.
Being in New England, lets say I buy a some land where unknown, historically insignificant settlers made an attempt at building a homestead in the early 1500's. Lets say that I find some partial remains of a buried rudimentary mud/brick/wood small building. Lets also say that the remains of a few humans were found on site. I'm sure the rules say an archeologist would have to be involved but in the end of the day I'm sure excavation would eventually continue and the remains would be "put to rest" somewhere else.
Now lets say the same thing happens in Greece but what is found is 2,000 years old. Happens quite often actually. They do a proper investigation and excavation and then things eventually move on. Same story but it probably takes a bit more time.
Where it gets tricky is when it is of historical significance. For example if a previously unknown historical structure, person or wealth is discovered. Most countries have very clear laws on how to handle this and many times it is not the current property owner that gets to keep any wealth that is found. They get transferred to the government and if you are lucky you get fair market value compensation for what is found.
But in the end....what is the difference between moving a small 1600's cemetery in Massachusetts to make room for a new parking lot vs moving some aristocrats from a crypt in medieval Europe?
The difference is when things are done purely for profit without regard for the historical significance. For example lets say the year is 1900 and some expedition of men go to Egypt to break into a structure using explosives to find the wealth hidden inside. In the name of discovery and archeology they tunnel into the structure and find all the treasures and sarcophagi they hoped for....then they crate it up and bring it back to the Museum of London, leaving behind a partially destroyed historical structure.
I'd say that is where the line blurs and they crossed into grave robbing territory. Thankfully this doesn't happen as much by western civilization anymore. Still happens plenty though in many parts of the world....