The reason why the appendQueryString is set to true is because its semantics in IIS is somewhat different from QSA in mod_rewrite.
In mod_rewrite if substitution string does not contain query string then the original query string is preserved regardless of QSA flag. For example this rewrite rule:
rewriterule a b
will replace "a?c=1" to "b?c=1".
In IIS URL rewriter if appendQueryString is set to false then it means that original query string will never be preserved. For example this rewrite rule:
<rule> <match pattern=”a”> <action type=“Rewrite” url=”b” appendQueryString=”false” /></rule>
will replace "a?c=1" to "b". Notice that the original query string will be dropped.
So in order to get the exact same behavior of the converted mod_rewrite rule in IIS the rule importer tool sets this flag to true if the substitution url in mod_rewrite rule did not have the query string.