I rule it as written, because I usually don't want to bother trying to differentiate between the various fluff reasons why people get pushed/pulled/slid.
I can see this point of view, but... to me, the RAW result is highly counterintuitive, and I see this as a problem. I'm not saying the rules should
never produce counterintuitive results, but they should not do so without a damn good reason, and I don't think there is a good enough reason here.
Let's say a monster tries to slide Player X through Player Y's square. Player Y will naturally object to this; she doesn't want her ally moved through her square to a disadvantageous location! If I go by the RAW, then I have to tell her she can't stop it, and either come up with some shaky in-game justification or simply resort to "them's the rules"--both of which I feel are disruptive to immersion. Then next round, when Player Y wants to retaliate and slide a monster through Player Z, I have to do it again. It breaks immersion
and it's disempowering to the player.
The way I see it, you should always have a choice whether to let someone through your square or not. That gives players more control over stuff within their reach and increases the sense of connection with their characters, as well as being a lot easier to justify in-game. As far as I can see, the main benefit of doing it the other way is that it's in strict compliance with the rules as written. That
is in fact a benefit... but for a corner case like this, not a very big one.