It's less regulated so it's more likely people will make poor trades - maybe it's not in their interest as you are implying, but they do it anyway.

Countries can make owning foreign currency illegal. BTC is easier to hide than Euros (cash or electronic). You can trade BTC back to cash without a paper trail by writing down a private key.

In terms of why someone would sell BTC for a failing currency? Maybe they are also a local and have expenses to pay. Otherwise, it's likely the seller is selling for USD or something and there's an intermediate conversion so someone else who needs it ends up with the local currency. It's pretty common to get out of the currency as much as possible (assets, other currencies, BTC) then only dip back in for as little as possible to pay local expenses.