I notice that people are getting banned from this forum for posting low-content
It happens
very rarely.
however I also notice that some other people who post low-content don't get any bans. Is it because the rules apply to some and it does not apply to others?
No, it's because the rules are designed in such a way that a ban is an absolute last resort. If you have a look at the rule in question, you will notice that it is explicitly quite lax:
6. Avoid including material which does not contribute to the point you are making
This is a fairly vague rule, and as such, it will not be strictly enforced on a per-post basis. Rather, members who repeatedly make posts with a large amount of extraneous material will be asked to stop, and if they fail to do so, given a warning. After no less than three warnings for this offence, members may be restricted from posting in certain fora in order to prevent their verbosity from cluttering up debate threads.
Most people get warned a few times and then cut it out. Others get warned and choose to take it as a challenge. If you look, for example, at
neutrino's recent posts (no idea why I'm bringing him up in particular, clearly just a random selection on my part), you will notice that he is not a contributive member who occasionally makes the odd off-topic quip.
Most of the stuff he says is off-topic quips, and when the offending threads get moved to an appropriate forum, he just creates new ones.