Indeed, it must have legal value. Even so, some stores accept Bitcoin and Monero (thus making them money) but won’t accept USD in Europe or Euros in the US, so

Indeed, it must have legal value. Even so, some stores accept Bitcoin and Monero (thus making them money) but won’t accept USD in Europe or Euros in the US, so you can't use them despite those currencies having legal tender status. So yes, whether Bitcoin is money or a security remains an open question.