When you use these datasets you pay for how much the query has to scan. For example, if you do a select on a single 8 byte column with 1 billion rows in the table you are paying for 8GB.
Every month the first TB is free but if you go over that you have to pay $5 per TB. They are not exactly doing this for free.
Still doesn't mean Google will maintain the public access to this database. They might just as well shut it down and limit its access to Googlers. Like they already did a few years ago.