Note that there is a big jump between something being considered for being against the rules and for any occurrence of it to be called cheating.
Let's look at some of the things that are against the rules:
patterned or non-Pokemon art back sleeves: There are two reasons that this is against the rules. One is that some artwork can be inappropriate and while some cases are obvious (nekked ladies), there are a lot of edge cases that POP did not want to put their TOs and HJs in the position of having to decide on. Also, patterned sleeves or some artwork can make marking cards easier, as subtle differences in a design or pattern can be used that are very hard to detect. This is not saying that everyone who would use non-Pokemon artwork sleeves is trying to subvert the morals of young players or trying to mark their cards. The vast majority of players have no such intention. But the rule was put in place to protect against those few cases.
Similarly, those "declumping", for the most part, are not trying to cheat and will shuffle their deck sufficiently to randomize it. However, there was a concern that it made it easier to cheat (either intentionally or even unintentionally) and so it was considered for banning under the rules. However, as has been touched upon quite a bit in this thread, discerning the difference between declumping, temporarily making a possible choice, making an actual choice, but not revealing it yet, then changing one's mind, and many other similar actions would be next to impossible and so enforcement would either be impossible, or impossibly heavy handed.
It is not necessarily saying that all actions that move cards around equals cheating.