Because the manga didn't specify:
Burglary: Entering a dwelling with the intent to commit a felony once inside.
Robbery: Taking another person’s property without their consent with intent to deprive him of it permanently, and the taking occurs from the victim’s person or oi his presence either by violence or putting the victim in fear of imminent physical harm.
Those are the legal definitions of each crime, at least in the US (may differ in Japan). So, the criminals are better defined as burglars than robbers, unless they were going to steal the family documents in the presence of Will while threatening physical force.