EDIT: Maybe I should have said that they don't prevent it rather than they "allow for it". I have not checked and am unaware if it goes against their TOS.
As Fozzy pointed out, both PT & HEM import Winning's observed hand histories because WPN provides them, this is considered a feature of WPN, and it is our jobs as software developers for the two biggest tracking tools available to support features that networks provide (assuming that support can be done of course, not everything can be supported). The important thing to note is that WPN management knows that dataming exists, and they know they could stop it if they disabled observed hand histories - since mucked hands are not available at showdown, it is our opinion that WPN management does not view datamining as a concern, last our legal team checked there is not issues with datamining in the TOS, and there should not be since WPN itself would be facilitating the ability to do this. Regardless of your position pro/con about datamining, you must respect WPN's management position on this topic, they take a much healthier approach to the issue than most other companies do.
Also, I seem to recall awhile back that HM and I think PT cooperated with sites that did generate HH's for observers by only importing 1 or 2 hands without sitting, for purposes of table selection. I don't know if PT4 is doing that with WPN or not. I suppose I or someone could test the theory, although I almost never play on WPN anymore.
This isn't quite true. We do not support hands from dataminers, we only import qualified hands from the networks that we support. If the network provides observed hand histories in any form, then we must allow support for those observed hand histories - for example
pokerstars has observed hand histories saved in your local computer when you are seated at the table but sitting out - we therefore must import these hands (An observed hand has no HERO therefore it is easy to determine what is and is not observed). Datamined hands are observed hands, therefore if the network provides observed hands, and the dataminer has successfully spoofed the poker room's observed hand format, then PokerTracker cannot tell the difference and will import the hands. This is why tracking software is not to blame for datamining, the networks themselves are... we cannot police this issue, but the networks can, and in some cases they have such as PokerStar's shutdown of PTR.
Earlier Fozzy discussed PokerStars prefetching; PokerStars has given us written permission to pre-fetch the first 25 hands of a session to help place the HUD when first sitting down - this is the only network that allows that however, its a unique feature of Stars. We have legal permission to do this, I cannot speak on behalf of other companies however but we would not have done this without permission first. Our policy is to always follow the Terms of Service, and respect the wishes of the network via business agreements.
I should note that as a company we are VERY anti-datamining if the network does not allow datamining, but take a neutral stance if the network permits it - thats the network's choice to make, not ours.