I don't mind having to pay for these updates if necessary. They should just be honest and transparent about what they are doing.
https://launchpad.net/debian/+source/imagemagick/8:6.9.10.23...
There are no later versions of imagemagick on ubuntu 10. So, my guess is that Ubuntu has (and will continue to) take any security updates that appear in the upstream Debian release, and add an Ubuntu Universe package for them. Now, I'm guessing, there will be additional security updates in the Universe package set for users paying for Ubuntu pro, where those packages are not available on Debian (i.e. Ubuntu themselves will package them).
If that's the case then there is nothing nefarious going on, just Canonical didn't explain it very well.