Because you can't sue a reseller for breaching a contract if you never had a contract with them in the first place.
http://en.wikipedia.org/wiki/Privity_of_contract#Exceptions
Also notice that this doesn't include prominent examples from labor law (company A that hires someone who will knowingly break a non-compete agreement they held with company B can be held liable even though company A wasn't a party to the contract (c.f. the MSFT/GOOG executive hiring war)).
Why would you not try and protect your supply of goods