I travel regularly, and worry if this can happen to me! If there's some risk, what to do? Call housekeeping for new dispenser bottles when I enter room?
Using refillable dispensers is already common practice in many parts of the world and everybody seems quite happy with it. It has started to show up in the US as well, and I've even seen just last night in my New York hotel.
In most cases the dispensers are mounted upside down on the wall and you dispense by squeezing the dispenser, pushing something up, or turning a knob/handle. It's not unlike the soap dispensers you see in many public rest rooms.
Overall it works well and is super convenient. It would be difficult to tamper with, but then again, everything can potentially be tampered with. However, the likelihood is extremely small and there is really no motivation to do it
You do realise that a single-use bottle of conditioner passes through quite a few people before it ends up in the hotel room, you're going to use, and any of those people might have replaced the contents with anything? So this doesn't really pose any new problems.
At the hotels I've been at in the last many years that have done this (Marriott is far from the front here), the dispensers have been closed, so it would take some effort to open it. I must admit that I never bothered to look so closely at it as to figure out how to open it. Furthermore they're placed such that I think you would have to be in a rather uncomfortable position to jerk into it.