I like the word "liability" you used here, because in Germany (where I've worked for the last ~5.5 years) there is another perspective of 'liability': an employer has to ensure that their employees take their vacation and adequately rest, else they expose themselves to legal liability. This isn't just about vacation - but also rests during each working day (I think it's 30 mins for < 9 hour workdays), and between working days (at least 11 hours between the end of day X and the start of day X+1).

I was travelling by bus and we got delayed. The bus stopped 15 minutes before reaching the destination because the bus driver had to take his 30 min rest. The rules are pretty strict especially for drivers.

so, the middle ground is, "unlimited" is not, at all, unlimited.