a) I've found that lots of design applicants will put stuff in their portfolio that they contributed to, rather than built end-to-end. This solves that problem.
b) Tracking on the statement above, they're looking for a designer that has a "deep understanding of how applications should work". In an online design portfolio it's easy to misinterpret application design skills w/the ability to take a design spec from a product manager and make it pretty. Stripe is looking for a unicorn - a designer that is also a product manager / UX talent.
c) Design, especially UX design, is not just a "show me your portfolio" - it's a "what is your process to build the right UX." If a candidate submitted several iterations of design + maybe a "let's test these two things because I'm not sure" I would be 100% more likely to prioritize their app vs a "look at how pretty this is"
So I don't know, I get the argument that this is taking advantage.
But on the other end it makes them a lot more likely to find an awesome candidate who actually wants to work there, and has the talent to deliver the goods.