Adding to that: one of the fundamental EU rules was freedom to do bussiness anywhere in EU. That worked in favor of Western countries which had their corporations opening offices in Poland, and making huge profits here.

After some time Poland found some niches for itself, e.g. Polish transportation companies dominate EU market in truck freight. What was the EU response to that? They recently passed change in labor laws, that would make this business unprofitable.

So: common EU market is fine as long as Western countries benefit from it, but Poland or other Eastern countries making money from business in the West is against the rules :)