Lots of good points here. I have no engineering experience so I can't talk shear vs tensile applications. But I can add a bit to the military side of the history. jmann correctly points out the use of domes on the Colt commercial stuff. I have the remnants of the Colt rivets, including the large trunnion ones, on my 1928 kit. What I am hoping to figure out is how to accurately press a big dome like that when I build the bugger. Probably going to need some good heat on that one!
As for military production, all the rivets were chamfered and ground flush on the WWI 1917 production, including the cams on the LSP. As guns were rebuilt into 1917A1s, and 1919A4s were produced, there is some variety. The cams all have domed heads on new production. I have examples of 1917 LSPs where a cam has been replaced, so one is flush and one is domed, on conversion to 1919A4. Saginaw tended to use domes of about the standard size on the top plate at both ends, but I have seen a pic of one (appears original) where the rear is domed and the front flat. RIA uses a pretty large diameter, but short dome on the rear, and a smaller dome at the front. I have not seen conclusive images on BA guns, but my guess is they tended to domes like SG.
No question that the pan head with the ground flat, for inside the rear top plate, was the standard on every version of the series. I think the points by msg and alcova explain the most likely reasons. And at Dan to that, who slipped in while I was typing!