The Bitcoin price is determined by demand and supply. If people want to invest in a HYIP they need Bitcoins, so demand goes up. This results in a price increase.
Back in December 2011 the Bitcoin price was at a local minimum around $2. BTCST was started at about the same time, and we had a constant upward trend since then. This was not only due to BTCST (e.g. SatoshiDICE was lunched in April), but BTCST contributed to the upward trend.
Unless pirate was selling the coins, then investment in BTCST would've constituted supply, not demand.
I also doubt many of pirates investors specifically bought bitcoins to invest with him... anything to back this up?