Some reasons have been provided by Rep. Castle himself as he attacked a bill to increase the funding for research which explores how to obtain pluripotent stem cells without killing human embryos. Richard Doerflinger also points out Castle's confusion.
In other stem cells news, this morning I saw an approximately 3 minute segment on Good Morning America where Michael J. Fox laid out his reasons for supporting embryonic stem cell research even though the words "embryo" or "embryonic" weren't in either the intro or anything Fox said. Fox also acted like we lost 5 years of research, without mentioning the research is legal and already receives federal funding.
No one opposed to increasing the federal funding of embryonic stem cell research was given a word on why they are opposed to the increased federal funding of this research.