Many websites now request that you use your Google + or FB account to access the site. However, they still are required to have another option for you to use to access the site. This normally requires that you give them your email address and create a password. The email address will be checked and verified before you can use the site.
Today people don't think twice when they use their FB account a website. What you are basically doing is giving this site permission to use your timeline to post what eve5 they want. In some cases, they can spam your timeline with tons of ads. You are giving them free advertising space on your FB account.
I know that they can access your personal timeline, contacts, and other information you have on your FB account. I personally, don't like to give anyone access to my FB account when joining a website. I don't think it is really safe to allow people to post what they want on your timeline.