2. Based on my research, those accounts already got banned. Why does moderators ban those accounts? it's temporary or permanent? AFAIK most accounts got banned due to plagiarism or evading ban, not spamming.
You are not most likely going to get banned for spamming a single time. If you spam a lot, you will get banned of course. The above mentioned users have no contribution on the forum, most of their post are spam. It can be safely assumed that they will only bring spam to the forum, nothing else. Having them banned makes sense I think.
Yes you are right, spammers that is not contributing anything meaningful to the forum is supposed to ban because the reason the forum is created for good communication and also bring a solution that will solve big problems. But some of us don't read and don't take the rules and regulations serious, so they are supposed to be ban. It's in the law of the forum that pointless contribution is not supported and any pointless response is as off-topic, so if what they will be given is off topic let them be ban.