Well, actually, you don't submit your Yahoo info to my site. Here's how it works:
Option 1: Enter your league information on the homepage (league ID, sport, year). You will be taken to the Yahoo site itself, at which point you enter username/pass. My site is then authorized to read from your Yahoo leagues, which is how the standings are calculated.
Option 2: Create a separate account on my site, go through the same Yahoo authorization process, and have all your leagues loaded automatically.
It works using the Yahoo Fantasy Sports API, which was created for websites like mine. At no point do I ever collect personal information, which is indicated on the Yahoo authorization page.
If you still feel insecure, then I'm not pushing anyone to use it. Like I said, I don't stand to gain anything. I created this for personal use (computer nerd + fantasy nerd), but figured I could expand it to a wider audience.