Any website that complies with PCI Data Security Standards has to adhere to sections
- 8.5.13 (Limit repeated access attempts by locking out the user ID after not more than six attempts)
- 8.5.14 (Set the lockout duration to thirty minutes or until administrator enables the user ID).
This is unfortunately why a lot of sites accepting credit cards have draconian lockout policies, even though their designers may not necessarily agree with what they've implemented.
Edit: Note that these requirements only apply to systems for "non-consumer users" so they shouldn't affect customer sites accepting cards.