Why are UPCs / EANs not imported from Amazon?
Unfortunately Amazon doesn't allow seller to use the MWS API to fetch the Product ID (UPC / EAN) for a specific ASIN or SKU.
According to this post in the Amazon Seller Forums, "barcodes (so UPCs, EANs, etc) are considered proprietary information" - just like product descriptions and customer reviews - which is why they don't make this available to sellers via their API.
That being said, the MWS API is not the only API provided by Amazon - it's the one used by seller to manage products and inventory though, and it's the only API used by WP-Lister for Amazon as of now.
For associates / affiliates there is the Products Advertising API for Associates, which apparently would allow to fetch the UPC / EAN for a specific ASIN - but it only allows a limited number of requests per day and requires you to become an associate, something you would usually sign up for to embed affiliate links on your site (and earn commission by driving traffic to Amazon).
WP-Lister for Amazon will eventually support the Products Advertising API as well, but this will be implemented further down the road - possibly in version 1.5 or 2.0, but not in 1.0. If you are a developer who would like to work on this, please contact us.