Hello Vorpal,
Yes, I noticed, but it seems to be more common at 0.10/0.20 than say at 0.25/0.50 where the betting does get capped quite frequently.
I'm not an expert but imo the fact that they are not capping predraw with small trips is a mistake. Why? Because if you hold trips, the chances are simply too high that you hold the best hand predraw at a 6 max table, even if someone raised in front of you (which he may do with AA, a two pair, etc.) And if you hold the best hand predraw, you are often a huge favorite in 5CD since there is only one more round to come (the postdraw round). Being probably a HUGE favorite predraw (it can be as high as 92% depending on what your opponents are holding, for example 777 is a 92.1% favorite over 3322X) you just have to get as much money as possible in the pot.
Also, if you slowplay trips predraw, you are giving others a cheap chance to call with say a two pair and draw out on you by making a full house. Whereas if you raise with them, maybe you drive that lousy two pair 3322 out and you deny them the chance to beat what is usually the best hand predraw.
Now, that's the "general idea", but of course your best play always depends on your opponent and how he plays. If you observe him and you notice that indeed, he only 3-bets when he holds at least trips, and if not, he just calls, you have to adapt accordingly and fold after his 3-bet when you hold lower than trips and maybe just call with trips. I must say I haven't seen this kind of player yet, but if you're sure his 3-betting range is limited to trips or better, you have to adapt your own ranges for calling the 3-bet or capping to that imo and maybe just call with low trips and forget about capping. There are situations in which it makes sense not to cap trips, and this is probably one of them.
I think the reason why many players at 0.10/0.20 never seem to cap is simply because they do not understand how big a favorite they are with trips. Sure, sometimes it happens (especially at PokerStars lol) that two players are dealt trips predraw, but this does not happen all the time or every day. I think on average you'll win a lot more with trips than you'll lose with them.
Postdraw is a different matter. If you hold say 666 but still have 2 or 3 other players with you, you cannot fold imo (unless someone stood pat and you think it is legitimate) but you have to be careful and not cap again postdraw.
To be honest, I notice the weirdest things currently in online 5CD. At 0.25/0.50, there seem to be only nits who always seem to hold at least AA, AAXX or trips, and at 0.10/0.20, as you mentioned, you see some weird and very unusual plays such as limping in even with AA or two pair, not raising (let alone capping) with trips etc.
Yesterday I saw a player who held a full house postdraw against 2 players drawing 3 and did not even value bet. And actually a month ago a Bulgarian player said in the chat that I was an idiot because I played inferior hands like QQ, JJ or TT, raising first in (which of course I raised from cutoff, button or SB, not from UTG or HIJ) and then hoping to get lucky