The only options they have is single seat or "talk to us" Enterprise level and for basically a one-person company using a virtual assistant we pretty much fall through the cracks there.
Shutterstock should care how much OP paid, because if they were paying a subscription and not maxing it out, Shutterstock lost money because they had a draconian TOS that they aggressively enforced to their own detriment.
What is your strategy for all you can eat buffets?
Your comparison is actually more like if you just ordered a regular meal and then shared it between two people which is allowed...
Also I should note that users are a useful concept for businesses so that they can manage usage and things like access control easily but for me I didn't need that so a single user was sufficient.