Reminds me of Mastercad's lawyers pulling similar crap:
Mastercard's threating letter is here:
http://www.netfunny.com/rhf/price.html
And the guys response:
http://home.cc.umanitoba.ca/~umroyma...ard/mcrhf.html
Basically, they really have NO legal justification for their demands about 3rd party links... but it's good that you responded just so you don't fall into any technically legal bullshit traps. Of course, even a simple "Go Fuck Yourself" would've constituted a response no?
If you're making money, there will be different rules concerning the usage stuff, but even then I highly doubt those sharks have many (if any) in their favor.