Not being able to send means they can't use Starlink in Belarus, right? There's no ability to send e.g. HTTP GET, so you can't actually use the internet. Right?

correct. last time i checked, sat internet is possible, but outbound connections are happening via local ISP. This worked quite well in the past to download large files. Nowadays with fast local internet such sat/local setup doesn't make much sense anymore.

how would they govern the sending to sats?

via starlink themselves?

Starlink offers full duplex lines compared to classical satellite connections that require a secondary line for sending data.