Thanks, I have removed you from the blacklist now!
Also, are there any guidelines for how to use steem?
You should check out the Steemit FAQ here: https://steemit.com/faq.html
Why can they do it? What makes it no spam then?
This is considered a bit of a grey area. These Dapps are generally doing it on their own users or are opted into it. Or they are doing it to new users the one time. There have been instances of them going out of bounds and we have responded to these instances and they have stopped. They also leave an upvote, which I guess can be considered as a payment to the user for commenting on them. A few of the Dapps have an opt out feature too, which is something we like to see.