The fact that the SBC did not invite the PBU suggests that they are not being considered for their alternative Baptist World Alliance.
All I know about the Polish Baptist Union is that they are affiliated with the Europian Baptist Federation and that they are part of the ecumenical council of Poland (together with the Lutherans, the Calvinists, the Methodists and the Eastern-Orthodox Church in Poland), that's probably evidence enough that they are evil(TM) in the eyes of many.
As for their link with Communism.
Polish Church Celebrates 125 Years
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For many churches in Europe, celebrating a 125-year anniversary is almost commonplace. However, for the Baptist Church in Lodsz, Poland, it is a great accomplishment due to the fact that for many years the Polish Militia, the counterpart to the KGB, used the building as its headquarters.
After the fall of Communism, many church members never believed that the building would never be returned to them and community members were convinced they were fighting an uphill battle.
"Many people asked us how we could ever dream of taking our property back from the police," said Leszek Wakula, president of the Polish Baptist Union. "They said we were crazy."
The struggle to regain the church was, of course, very difficult and time consuming for leaders and members.
"The government even went so far as to send a building specialist to the site to see if there was any evidence that it had ever been used as a church, even though we had all of the plans and documents," said Wakula. "You can only imagine the inspectors surprise when he found the baptistery under the stage. This was sure proof that it was our church."
During the anniversary celebration held 20 September 2003, one guest told of many years before when he had attended a communist rally in the very same building.
"It is hard for me to believe that after 20 years I am standing in the same building that was at one time the headquarters for my work and my life," said the now Christian attendee. "It is wonderful to be standing in a place that has been given back to God."
"We have much older churches in many places of Europe," said Theo Angelov, general secretary of the EBF. "However, we don't have many that were used as communist police headquarters. This anniversary celebration is even more evidence of God's ability to overcome oppression."
http://www.ebf.org/bulletins2003_11.htm