>Why would they expect this account to be used as a high-velocity, low-balance account rather than a park-your-savings account, given the rate?
Think of it like being a health insurance salesman who puts your office at the top of a long flights of stairs. You can have a lower risk pool because the only clients who come to see you are the ones who can climb a bunch of stairs to get there.
In other words, they advertise to a user-base that doesn't have much in the way of savings so they're avoiding people who would keep high balances anyway.
Besides, most people who do have large balances tend to park them in money market funds anyway. They can usually hit between 2% to 3% annually and offer more flexibility in terms of being able to access the money without transferring balances around. Alternatively they'll be putting it in tax-deferred accounts.
If you have enough money coming in that you can accrue a lot of savings, a savings account isn't the most productive place to put it. It should be regarded as more of a rainy day fund or a place to park money that you're saving up for something specific, like a down payment.