To be fair, visa-free entry into Belarus for up to 30 days is a completely new thing[1]. Unlike most visa-free regimes, it's not driven by any international agreements, but rather by a Belarusian initiative to promote tourism in the country. Therefore, they can set their own requirements, and it's still not possible to take a 2-hour train from Vilnius to Minsk without a visa. The entire program can be cancelled at any time by one side, since Belarusians don't have the same visa-free access to the EU.
To be fair, Ukraine has the same unilateral visa-free regime for EU (plus some other countries) citizens. But it's not as arbitrary. If you are a citizen of one of the countries you can enter through any legal port of entry.
Edit. I keep forgetting that Ukraine now does have visa free travel with Schengen states.
[1] https://www.belarus.by/rel_image/6400