Shabbos 41 – שבת מא

The Gemara differentiates between covering oneself when facing the public, on account of modesty, whereas on his way down to the river that is not allowed.  Since the concept of tznius doesn’t necessarily depend on presence of other people, but rather on Hashem’s presence (See Shulchan Aruch O”Ch 2,2); if there indeed seems to be a tznius concern here, why wouldn’t this apply in any circumstance?