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Burrowing is just digging, like using a shovel very quickly.
If a critter can only dig through dirt, I can see wood being a problem for them, but if they can dig through stone, wood's actually easier than they are used to.
You know, you are right. But sometimes rules are there for good gameplay and not to totally represent real world physics exactly.
As an example, you know what else is easier to dig through than stone? Soft yielding monster flesh. If a creature can burrow through stone, surely it should be able to burrow through monsters and auto kill him, right? (or at least, let it burrow through a stone golem and auto kill that, right?).
But thats not how the rules work. And there are some good reasons for it, even ignoring silly scenarios like burrowing through creatures. For example there are high level spells like Earthquake for destroying the wooden fort or the king's manor, that would get obsoleted if you could do the same thing with a badger summon/pet at level 1 and at totally unrealiztic speed.
Now it seems you already decided what answer you wanted before you started this thread, and if so, I would say go ahead and houserule it (did someone summon a tree to stop a monster or something? Its fine to use common sense saying that a little tree wouldnt really bother a giant monster, I dont think anyone would argue too much if you handwaved it for a purple worm even). Anyway, though, you know what RAW (such as it is) is, now, and always it is best to know RAW first before going to the house rule.
Hope htis has helped.
EDIT: Also, for roots and such, this is where GM is supposed to use his judgment to decide whether roots are an impediment or not. Some tiny little plant roots might not stop a purple worm, but mighty jungle behemoth redwood roots a yard thick might slow it down. You can't expect a rulebook to cover every little situation like this and spend pages telling you exactly how thick a root is big enough to stop a burrowing purple worm for the one in a thousand groups who this would matter for, this is what GMs are for.
And I think we only offered that interpretation as advice to you in your GMing, as it seemed you were looking for a way to rule on a critter going through roots. That is how I would do it. But that's more offered as a way to make RAW (you can't burrow through wood) and your complaint about creatures moving through roots less disagreeing with each other, wile still being a common sense explanation. So if you hate it, don't use it. I will say though that it is totally silly to insist on RAW on roots slowing something down (which seems like good common sense) but then say that common sense has to overrule RAW with burrowing through wood at all. Pick one or the other! :p