Why is http://sitename/web.config downloadable on D7 sites, and what is it?

This doesn't seem to happen with D6 sites as far as I can tell.

Contents of the file:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <system.webServer>
    <!-- Don't show directory listings for URLs which map to a directory. -->
    <directoryBrowse enabled="false" />
    <rewrite>
      <rules>
        <rule name="Protect files and directories from prying eyes" stopProcessing="true">
          <match url="\.(engine|inc|info|install|make|module|profile|test|po|sh|.*sql|theme|tpl(\.php)?|xtmpl)$|^(\..*|Entries.*|Repository|Root|Tag|Template)$" />
          <action type="CustomResponse" statusCode="403" subStatusCode="0" statusReason="Forbidden" statusDescription="Access is forbidden." />
        </rule>
        <rule name="Force simple error message for requests for non-existent favicon.ico" stopProcessing="true">
          <match url="favicon\.ico" />
          <action type="CustomResponse" statusCode="404" subStatusCode="1" statusReason="File Not Found" statusDescription="The requested file favicon.ico was not found" />
        </rule>
        <!-- Rewrite URLs of the form 'x' to the form 'index.php?q=x'. -->
        <rule name="Short URLs" stopProcessing="true">
          <match url="^(.*)$" ignoreCase="false" />
          <conditions>
            <add input="{REQUEST_FILENAME}" matchType="IsFile" ignoreCase="false" negate="true" />
            <add input="{REQUEST_FILENAME}" matchType="IsDirectory" ignoreCase="false" negate="true" />
            <add input="{URL}" pattern="^/favicon.ico$" ignoreCase="false" negate="true" />
          </conditions>
          <action type="Rewrite" url="index.php?q={R:1}" appendQueryString="true" />
        </rule>
      </rules>
    </rewrite>

    <httpErrors>
      <remove statusCode="404" subStatusCode="-1" />
      <error statusCode="404" prefixLanguageFilePath="" path="/index.php" responseMode="ExecuteURL" />
    </httpErrors>

    <defaultDocument>
      <!-- Set the default document -->
      <files>
        <remove value="index.php" />
        <add value="index.php" />
      </files>
    </defaultDocument>
  </system.webServer>
</configuration>