What does being acknowledged in a medical journal imply?
From the Lancet:


  Acknowledgments — written consent of cited individual


From CMJA:


  Authors should specify, in the acknowledgements section, contributions to the paper that should be recognized but do not justify authorship, for example critical review of the study proposal or assistance with statistical analysis. The Canadian Medical Association Journal (CMAJ) requires that these people give their written permission for their names to appear in print.


From Infection Control and Hospital Epidemiology:


  Thank you notes. Persons should not be thanked in the Acknowledgments
  section without their knowledge and consent. Authors will be asked
  during the submission process to confirm they obtained permission from
  all persons thanked by name in the Acknowledgments section.


I suspect some of this is from the very defined notion of what constitutes authorship in medical journals, and the likelihood that someone who had some impact on the reported study would not make the authorship list. For example, the same ICJME that has developed the most widely used authorship standards also has this to say about non-author contributors:


  Because acknowledgment may imply endorsement by acknowledged
  individuals of a study’s data and conclusions, editors are advised to
  require that the corresponding author obtain written permission to be
  acknowledged from all acknowledged individuals.


Basically, the assumption is that, by being acknowledged, there is the possibility that you could be seen as endorsing the findings of the study, and that erring on the side of caution, you should have to obtain permission before acknowledging someone.

This is even true for not medically-specific journals. Consider, for example, this entry from PLoS One:


  Authors are responsible for ensuring that anyone named in the Acknowledgments agrees to be named.