UASF is just any random node throwing a comment into the useragent but still waiting for weeks after 'activation' to actually get an implementation that does anything more than the tier network.
Thats exactly what BU is doing too. There is no activation code or threshold for BU. There is a suggested threshold of 75%, but that is not final and the current implementation has no activation code, so everyone will need to upgrade when that code is released. BU miners signalling is pretty much the same as the current uasf uacomment crap.