Chezkas Mamon (II)

The gemara at the beginning of Elu Metziyus (B.M. 21) raises a question of whether one is obligated in hashavas aveida in certain cases, and leaves the safek unresolved. The Rosh paskens on these cases that safeika d'oraysa l'chumra and one is obligated to announce the avieda. Why would we not treat this issue as a safeik mamon and say that the finder has become the muchzak in absence of any demand to return it? The Kuntres HaSefekos (6:9) suggests here as well that we only apply chezkas mamon where the issue at hand is a safek in metziyus. Where the issue is one of sadek in din, we apply sfeika d'oraysa l'chumra. (See the Rosh in perek 2 of machshirim re: rov yisrael).