After trying to find a solution in the dashboard, the way I solved this was to dig into the php code of my site.
In my case (using the Mystile theme), the appropriate file is found here:
Find the code that applies to your terms checkbox. Mine looks like this:
<p class="form-row terms">
<label for="terms" class="checkbox"><?php _e( 'I have read and accept the', 'woocommerce' ); ?> <a>" target="_blank"><?php _e( 'terms & conditions', 'woocommerce' ); ?></a></label>
<input type="checkbox" class="input-checkbox" name="terms" <?php checked( isset( $_POST['terms'] ), true ); ?> id="terms" />
Then, right behind
and save the edited file. The "agree with terms" checkbox should now be selected by default.
Hope that helps!