To be honest you are more aware about carders techniques than I am, I'm not into this kind of things at all, too risky and immoral. You can use them without any worries, trust me.
I only mentioned this as I have previously worked for a payment processing company and have seen this happen quite often (98% of online credit card fraudsters worked like this). Carders would use automated bots to check lists of stolen credit cards by purchasing trials on porn sites to see if any are still working, and then they would proceed to make bigger charges on the cards that work, and would usually sell the list of porn site logins in order to make some extra money and also to help cover up their tracks.
The only reason I express concern is that if any of these were purchased using stolen cards and somebody logged into them, their IP address would be given to credit card fraud investigators and cause everybody involved a whole lot of extra hassle. Again, I'm not saying these were purchased using stolen cards and want to make everyone aware that this happens quite frequently.
Nah just a dictionary cracking system for existing accounts, it looks like. Good point about the IP anyway though.