Subversion treats tags as branches; they are all copies of a certain trunk snapshot. In Git, on the contrary, branches, and tags have different significance.
To convert Subversion tags' branches into Git tags, here's a simple script that does the work:

$ git for-each-ref --format='%(refname)' refs/heads/tags |
cut -d / -f 4 |
while read ref
do
  git tag "$ref" "refs/heads/tags/$ref";
  git branch -D "tags/$ref";
done