I'm not saying you're one of these people, but there's a general meme of non-financial people who don't understand that justifications for raising capital can indeed be to liquidate investors (or founders) positions in companies. This is very normal. Investors should know (I believe they're legally obligated) what their capital is being used for. If they don't believe their capital will increase the value of the business, then they simply wouldn't invest.
The market should correct itself in this case (if not leaving enough cash in the company is a problem).
For SHOP specifically, 99.9% of the Class B stock (i.e., stock outstanding pre-IPO) is locked up for 180 days. (See underwriting section of the prospectus)
http://www.sec.gov/Archives/edgar/data/1594805/0001193125151...
On top of that, I imagine it's a little tricky because an IPO is when the early investors cash out, so they're the largest stake-holders in the company, and creating that "pop" in the hope that people buy-buy-buy to drive up the price creates a good return for those investors/stakeholders. I don't like it, since it can essentially screw the company out of cash, but that's my guess as to why such a thing exists.
On the other hand, a decent number of companies going public these days are going to screw over the investors anyhow (cough Zynga, GoDaddy, King), so if that means they get slightly less cash in the bank, I guess I don't really care.