There are a LOT of scams because there are a LOT of scammers ...it's that simple. And they live in all part of the globe. And many of them have absolutely nothing else to do ...
It's not just a Trulia issue - they are on HotPads.com, Craigslist and any other internet site that allows consumers to post ads. Trulia is VERY aware of the problem but is in a quandary - they want the public to be able to post rental ads like the other rental websites. In reality, especially here in the SF Bay Area, VERY few rentals go through realtors and the MLS, so if Trulia limits entries to those that only come through Realtors and the actual MLS, then there would be virtually no rental listings here at all.
Additionally, because the postings are free, there is no income to provide a paid monitor on Trulia's side of the fence - thus, just like all the other websites with rentals, they let the site be monitored by those who actually use the site.
Trulia has tossed around a number of ideas as to how to control the situation, but so far does not have a viable solution that is not fee-based.