It's the part about throwing away the last segment on the SNN ticket -- not the purchase of the one-way -- that may be a violation of the rules. Buying the one-way on AA could well draw attention to the fact that the OP skipped the BOS-LAX segment five days prior, and give AA a reason to collect any fare difference between a LAX-SNN round trip and a LAX-SNN-BOS open-jaw.

Mike