Daf Yomi - Yoma 46 - Zerika on Shabbos - What's Wrong?

Daf Yomi - Yoma 46 - Zerika on Shabbos - What's Wrong?

It is clear from Rashi and Tosfos on this daf that a possuk is necessary to teach us that shechita and zerika (the sprinkling of the blood) is permitted on Shabbos. Reshash is bothered as to what the issur would be to perform zerikas hadam on Shabbos? The Rashba in Menochos (21) seems to indicate that the predicament would be that the azarah is considered reshus harabim. The Mishna in Tamid states that if a sheretz was found in the azarah on Shabbos, they would turn a pot over it. The Rosh comments that the difficulty was that the sheretz is deemed to be muktza. He quotes others who explain that it would be forbidden to transport it on Shabbos for the azarah is reshus harabim. He disagrees vehemently with this, for the Gemora Pesachim states that it is regarded as reshus harabim only in respect to the halachos of tumah, but not for Shabbos. It would seem obvious that the azarah is not reckoned as a reshus harabim for it is surrounded by walls and the doors are locked at night?

Why would zerikas hadam be considered a violation of Shabbos, that we need a passuk to say that it is acceptable?