É porque esses clientes (atualmente) não suportam o OAuth 2.0 .
...beginning in the second half of 2014, we'll start gradually increasing the security checks performed when users log in to Google. These additional checks will ensure that only the intended user has access to their account, whether through a browser, device or application. These changes will affect any application that sends a username and/or password to Google.
To better protect your users, we recommend you upgrade all of your applications to OAuth 2.0. If you choose not to do so, your users will be required to take extra steps in order to keep accessing your applications.
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In summary, if your application currently uses plain passwords to authenticate to Google, we strongly encourage you to minimize user disruption by switching to OAuth 2.0.
Fonte: "Novas medidas de segurança afetarão mais antigas -OAuth 2.0) "- Blog de segurança on-line do Google