The reason is that Google has control over the XMPP server. They do not need to send you presence notification if your gtalk client is not in foreground. Just staying online is not the problem. The problem is that usually with XMPP the server sends presence asynchronously. There is no standardized way for a client to request not being send those. If you happen to have 200 or more users in your roster there is always someone going away or coming back. Receiving those packets is draining the battery.
If your XMPP server happens to support privacy lists you can request not being send those presence notifications for a while. But at least some servers do not send the last presence when you clear that privacy list. And at least some of those servers do not allow you to get presence for all users in your roster by sending only one (unaddressed) request for presence. You have either to send all users in your roster that request or drop the connection and reconnect to get the current presence of all users.
If your XMPP server happens to support privacy lists you can request not being send those presence notifications for a while. But at least some servers do not send the last presence when you clear that privacy list. And at least some of those servers do not allow you to get presence for all users in your roster by sending only one (unaddressed) request for presence. You have either to send all users in your roster that request or drop the connection and reconnect to get the current presence of all users.