I'm looking for a new SWE job in Europe (I am an EU citizen), preferably remote, and I'm a bit confused.

I come across a lot of job postings that say "remote in a country X", where the office, for example, is in Poland, but the position is fully remote, however, HRs insist that you must still be based in Poland (in this example).

However, you basically have to only visit the office once to sign the contract and potentially work from anywhere you want, or am I missing something? Is this just a weird choice by the particular companies or it has some legal ground I am unaware of?

I apologize if the question is stupid, I just wasn't able to google the answer to it. I'm also curious if I accept such a job offer and move to another country, could I possibly face any penalties?