There are levels of taboo and there breasts are nowhere near as taboo. Let's look at it this way-
Are breasts censored in Japan? no. Are genitals censored in Japan? yes.
Does regular anime shows, show breasts? all the time. Any genitalia? nope.
Do horror films show breasts? All the time. Do horror films show genitals? very rarely, usually if the film is not rated.
Can women go on the beach without a bra on? yes, women do it all the time. Can men and women walk around the beach nude? nope.
The only reason breasts are considered taboo is because for a long time our soicety deems it so and plenty of women these days are advocates in allowing breast exposure on facebook and other areas and that number still grows.
There are missing nipples in Senran Kagura because Japan is not allowed to show nipples in games otherwise they risk an AO rating and none of the three 1st party console developers allow AO ratings on their consoles. The Senran Kagura OVA has nipples so full bare breasts can be seen there. It's a really dumb policy and it makes no sense since the west has done it with God of war, Leisure suit larry (Which actually shows genitalia!), duke nukem, ECT. It's just a really dumb console policy in japan
I blame the Sonic fanbase on that one, too many furries and cringe OC's dwell in the darkest depths of despair.