IIS 7.5 forces the http header CacheControl to Private

In my .NET code, I have a custom handler that process Http request and in the ProcessRequest Method calls a custom HttpModule to set Http Cache Headers.

The HttpModule sets the header in the PreSendRequestHeaders method with the following code

HttpCachePolicy cache = response.Cache;
if (cache != null)
{
  cache.SetCacheability(HttpCacheability.Public);
  cache.SetMaxAge(TimeSpan.FromSeconds(varnishDuration));
  response.AppendHeader("Edge-control", String.Concat("!no-store, max-age=", akamaiDuration, "s dca=noop"));
}

In IIS 7.5, with the pool in integrated mode, the CacheControl is forced to private. Here is what I got :

curl -IXGET -H "Host:myHostName" "http://myServer/mypage"
HTTP/1.1 200 OK
Cache-Control: private
Server: Microsoft-IIS/7.5
X-AspNet-Version: 4.0.30319
Edge-control: !no-store, max-age=300s dca=noop
[...]

I don't understand why IIS changes the CacheControl to private.

Here is my webserver section in my web.config :

<pre>
<system.webServer>
    <handlers accessPolicy="Read, Script">
      <add name="OxygenHandler" verb="*" path="*" type="com.eurosport.oxygen.server.modules.OxygenHandlerFactory, OxygenServerModules" />
    </handlers>
    <modules runAllManagedModulesForAllRequests="true">
      <add name="WebServiceCachingModule" type="com.eurosport.toolkit.WebServiceCachingModule, Eurosport.Toolkit" />
    </modules>
  </system.webServer>
</pre>

I tried to add SetSlidingExpiration as mentioned here Cache.SetMaxAge not working under IIS, works fine under VS Dev Srv but it did not help.

I've managed to get it working with the following code in my module :

response.Headers.Remove("Cache-Control");
response.AppendHeader("Cache-Control", "public, max-age=" + varnishDuration.ToString()+", s-max-age=" + varnishDuration.ToString());

It looks dirty but it seems that response.CacheControl and response.Cache properties are not used by IIS in integrated mode (or are overriden by some module...)