Well, I woudn't worry about anybody using Kindle for a "free" internet access. First of all it's not really free - you paid for it.
But the real reason - all accesses go through Amazon proxies (yes, Amazon will see everything
). Sprint is filtering direct accesses on IP level (I get ICMP "Destination unreachable", with "Administratively filtered" code), and DNS doesn't even resolve anything except for Amazon servers.