Every baby is different. Just because one baby is able to sleep with noise doesn't mean all babies are able to do so. Just like some adults must sleep with a fan running and others can't sleep with a fan running. To state that every baby should be able to sleep through noise is unrealistic.
The OP has acknowledged that the parents were polite with their request - if not, by his own admittance he would have refused. Yet, he thinks that their bringing a baby camping (of which there are NO rules against anywhere) should not be allowed because he believes his right to use a generator all day long (of which rules against do exist in places) is more important. Maybe the OP should find some adults only campgrounds.