Sort:  

I'm new to all this tech too, but I think the answer to that question is that it is secure just because that's the nature (and the beauty?)of what blockchain technology is/does.

In the long term people using Steem/Steem Dollars as a wallet currency are going to have to adopt stronger security practices than just using a web site. That includes things like hardware wallets, 2FA systems, etc.

In the short term the web site is good enough for most people. For those with more value at stake, the command line tools can be used more securely, but require some technical skills.