You’re not wrong—all Beeminder goals are cumulative, which also means that they enforce the average over time rather than a daily minimum.
Your sleep goal is enforcing an average of at least 7(?) hours per night, and the same logic applies. It’s just that your daily number of steps are more variable than your sleep pattern, so it’s more visible.
For myself, a daily minimum of 10k steps each-and-every-day would be horrible. There will be days when I’m stuck in an office, or travelling, or the weather is horrid. As an average, I can walk more on good days and less on bad days, while still keeping on track. Even for more binary goals (did it, didn’t do it) we don’t recommend setting a slope of 7 days per week.
There is a ‘ratchet’ feature that can make today (or every day) an emergency day, requiring some steps, but even that won’t reset your daily progress to zero.