Yup - it's legal. Below from Wikipedia:
"The first-sale doctrine is a limitation on copyright that was recognized by the Supreme Court of the United States in 1908 (see Bobbs-Merrill Co. v. Straus) and subsequently codified in the Copyright Act of 1976, 17 U.S.C. § 109. The doctrine allows the purchaser to transfer (i.e., sell, lend or give away) a particular lawfully made copy of the copyrighted work without permission once it has been obtained. This means that the copyright holder's rights to control the change of ownership of a particular copy ends once ownership of that copy has passed to someone else, as long as the copy itself is not an infringing copy. This doctrine is also referred to as the "right of first sale," "first sale rule," or "exhaustion rule."
It sounds, according to this, that one is perfectly in one's rights to give away a pattern after purchasing it. If the original poster chooses not to do so in an effort to generate more sales for the designer as a show of support, that is certainly her choice, but it doesn't sound like she is legally obligated to do so...