Webmail is inherently insecure for several different reasons - one of which being that without SSL your correspondence is easily sniffed and possibly even stored in your browser's cache.
Using POP3 isn't much better in terms of privacy, which is why using POP3S to download your mail adds a bit of security that could keep the more likely snoops from reading your mail as it travels to and from your mail server.
Google is undoubtedly one of the largest Web mail providers today, and in case you didn't know it already you could access Gmail via SSL. However that's not the default mode of connectivity - which could present all sorts of serious trouble if people are accessing Gmail from open networks and public wifi hotspots where it's all to easy to listen in a vast array of digital conversation not meant for the public at large.
A group of researchers recently sent a letter to Google asking that the company make SSL connectivity the default mode of operation for Gmail. Google has now responded stating basically that they're testing and looking into the feasibility of the researcher's request.
One problem is server load from computational overhead. But since Google has a gigantic purse and a huge number of servers I doubt that supporting SSL by default for all Gmail users would present much a problem. At the same time, if they do make SSL the default connectivity method then other Web mail providers would need to follow suit in order to keep pace - which would be good for everyone that uses Web mail.