Because bydefault Membership uses these settings.
If u don't want these functinality u can change by setting in web.config
minRequiredPasswordLength="yourvalue" minRequiredNonalphanumericCharacters="yourvalue"
like....
<membership defaultProvider="SqlProvider">
<providers>
<add name="SqlProvider" type="System.Web.Security.SqlMembershipProvider" connectionStringName="LocalSQLServer" enablePasswordRetrieval="false" enablePasswordReset="true" requiresQuestionAndAnswer="false" requiresUniqueEmail="false" passwordFormat="Hashed" maxInvalidPasswordAttempts="5" minRequiredPasswordLength="6" minRequiredNonalphanumericCharacters="0" passwordAttemptWindow="10"/>
</providers>
</membership>