You are completely right, they are criminals there is nothing stopping them from just dumping the data anyway (or launching another attack later down the road).
However the hackers also want to get paid, as soon as they go back on their word no one else will ever pay them.
But there is another "maybe" to consider (OP did ask for a brief explanation so I didn't go into all possibilities), did they encrypt the data? If they did and entity no longer has access to it they then have two options 1) restore the data from backup (if they had them and can restore service in a reasonable amount of time) / write off any data loss 2) pay up for the keys.
