They do offer a free API but it is nowhere near as full-featured as WU. They have many tens of thousands fewer weather stations that report current conditions, for example. The variables returned are fewer when it comes to conditions and forecasts. Additionally, depending on your pay tier, WU had all kinds of historical info + specialized weather forecasts that NWS/NOAA does not offer. Even metrics like UV are rarely in NWS API products but could be expected from WU.

WU was also global, while NWS is US-only.

I love the NWS API btw. It's just nowhere near what the WU API was in terms of the promised data returned from API calls. NWS API has always been far superior when it comes to uptime!