If they have dysplasia (precancer) in them that is bad. If not then I believe the biggest danger is that it makes it harder to screen for areas of dysplasia.
They probably did not take them out because there was so many?? If you google pseudopolyps and ulcerative colitic you get some scary looking images, but it is the biopsy reports that are more important.
I am sure your doctor would of pointed out if there was dysplasia but just in case check the report. It should say something like "pseudo polyps with no evidence of dysplasia".