It's not that. Like most niche AWS services, this was likely the "pet project" of a major AWS customer that wanted something like this as part of a major business agreement [0]. And then after building it for that customer, AWS also expanded offering it to anyone else who wants to use it.
The overlap in work required to build AWS Ground Station and to build a production ground station network for a constellation like Kuiper seems fairly minimal [1], but it does seem like an ok way to get their feet wet while they don’t have any real satellites of their own flying.
1. Kuiper uses a different RF band (Ka vs X or S), needs an order of magnitude more ground stations, doesn’t need all the *aaS infrastructure bits)