Agreed, it is really nice to have the gmail SMTP server available on the road. However, it can be a little annoying:
1) messages are often rejected (immediate retry works 90%).
2) no matter what your "from address" or "reply-to" address is in your email client, gmail will override to your gmail settings.

There's a pretty simple explaination for 2).

If they used your current credentials, it would qualify as a relay. SMTP servers are big about not becoming an open relay and even though you can use SSL, most likely it's not required. So they're doing that just to make sure it's not a relay, which most likely would be thrown in your junk mail folder anyway since practically every spam filter known to man considers that spamming 101 (well maybe 102, but a remedial course at least).