When a product is free, it comes with a 16GB worth of daily "purchases" attached to it to prevent viral spreading of large files.
This means, for example, that 16 users will be able to get a product that totals 1GB, per day.
If you choose to pay any amount for the product however, the limit goes away even if it was reached for that day. Simply enter a number in the price field and the button will unlock.
*Note that products <100MB are not subject to this daily limit.