Mishna Para 10:3 begins
קָלָל שֶׁל חַטָּאת שֶׁנָּגַע בְּשֶׁרֶץ, טָהוֹר. נְתָנוֹ עַל גַּבָּיו, רַבִּי אֱלִיעֶזֶר מְטַהֵר, וַחֲכָמִים מְטַמְּאִים.
[The water inside] a chatas bucket that touched an impure small animal is tahor. If one placed it [the bucket] on top of [an impure small animal] R. Eli'ezer calls it tahor and the Chachamim, tamei.
The commentaries explain that the difference of opinion turns on interpretation of the words "וְהִנִּ֛יחַ מִח֥וּץ לַֽמַּחֲנֶ֖ה בְּמָק֣וֹם טָה֑וֹר" (B'midbar 19:9). I.e. the water must be placed in a "ritually pure place", and the dispute is what scope defines that "place". R. Eli'ezer holds that the container is the "place" and since it is pure the requirement is fulfilled, while the Chachamim hold the container's location is the "place" and since it is impure the requirement is missed.
While some commentaries extend the case of the mishna as a paradigm for other improper placements of the vessel, itself and its contents remaining otherwise pure all the while, the basic argument is somewhat surprising.
It seems that R. Eli'ezer is interpreting the requirement to simply be to keep the water tahor. According to the Chachamim, is the special rule of placement an additional requirement on top of the usual standards of tahara? This rationale is obvious as it could be argued that regular tahara rules are entailed by this higher standard. If this logic is not the basis for their opinion then what is?
(I could not find discussion of this specific point in the midrashim or the Tora T'mima.)