Commentary for Avodah Zarah 140:3

The story does not seem to be like a case where it is uncertain whether they entered a certain area. Rather it is like a case where we know they entered a certain area that had an impure thing there but we just do not know whether they touched the impure thing. After all, we know they opened many casks of wine. Thus even R. Eliezer should be stringent in such a case.
The answer is that there is in essence another uncertainty in play here. Since they opened so many casks, it could be that they were looking for money and not looking for drink. Therefore, this is like a case of uncertainty about whether he even entered and R. Eliezer would indeed rule leniently.