Using fossil fuels to grow food, move things and people around, etc. all puts CO2 and other pollutants directly into the atmosphere. Using it to make lego, on the other hand, is comparatively benign.

If they can come up with a replacement plastic that has a lower environmental impact, that'd be great. However, they shouldn't use a less environmentally friendly alternative just because it's not based on fossil fuels. That would just be greenwashing.