WHy is one end of DNA strand called the 5' "free" end, if it still has the phosphate group?

The 5' end is "free" because it will never ever be able to be elongated in that direction. DNA polymerases can only ever add new nucleotides to the 3' end of the DNA, and so potentially that DNA could be tied up if it was elongated. In contrast, the 5' end has a phosphate on it, and DNA polymerases cannot add nucleotides to that end, so it will always remain a "free" end.