Anyone who wants to can accept the card (technically, the certificate stored on the smart card). For example, I use a domain registrar who registers .ee domain

Anyone who wants to can accept the card (technically, the certificate stored on the smart card). For example, I use a domain registrar who registers .ee domains--thus requiring a digital signature from an Estonian ID--and can also use the card as a login mechanism. To be clear, the card isn't an ID in the sense of "here's my ID in person" (the card doesn't have a picture); it is a smart card with two digital certificates stored on it. More to the point, though, is that Estonia provides a digital identity and signing system that, among other purposes, allows you to access Estonian government services like forming a company. Since you can form a business under Estonian law, you operate as part of the European Union single market.