The fact that the offer at its minimum level (which is only free for 1 month, its an "up to 6 months" free deal) requires a commitment to maintain at least 1PB in the system for 12 months (which means you are on the hook for paying for it for the non-free portion of that commitment period), is probably a constraint on sending in junk data just for fun.

As is the fact that you have to migrate the data from non-Google servers -- either your own or some other cloud provider -- to qualify, since you're going to pay for the sending the data, junk or otherwise, you are dumping into it.