North Korea doesn't care because they are as sanctioned the west as a country can be - there's no additional leverage.

Russia is not in the same situation - they have been sanctioned, but there's many more sanctions that can be applied.

However, since they're most likely buying artillery ammunition and other weapons from North Korea, and they know that this traffic can't really be concealed, they have likely decided that obtaining this stuff is more important than avoiding whatever additional sanctions would be applied.