In practice this is less of a concern for a few reasons:

- They often designate items as "freeleech," which means that downloading does not cost you ratio, but uploading still credits you.

- They often provide free credits at various times, especially for new members.

- They tend to have a soft cap, so the automated warning systems don't even start until users dip below 0.9-0.95.

- As much as these sites have, there is always something they don't have, and providing it will get you a relatively high share of the total upload.

Bear in mind that the primary driver for the site is ensuring access to everything that is available.