The UK obviously is no longer a superpower. But the BBC is the cornerstone of the UKs modern global soft power projection. Broadcasting it free projects the UK government’s voice around the world directly into homes, influencing world policy to their liking.
Putting a paywall in the US sends a message that they feel it is not needed or not effective in the US market.
It also mirrors what paid sport broadcasting in the UK has done. Paywall it for short term gain, at the expense of long term viewership growth. The UK is struggling.