Tuvix's situation was unprecedented. Generally, when a transporter mistake occurs, it is (1) fatal, (2) does great harm to one or more people. Either way, correcting it if possible is pretty much expected. Here, we had a transporter error that was not only sentient, but didn't want to be corrected. If Tuvix had been the subject of "The Measure of a Man", I could see him being granted the protection of the law... but he didn't have it yet.
Consider this: if Picard had not fought Maddox in "Measure of a Man", and Data had died on Maddox's table, Maddox would not have been subject to murder, because (at the time) disassembling Data was not murder. Even if Soong type androids were later found to be sentient. This is because Maddox (and Janeway) were operating within the parameters of the law at the time. Many other evil and disgusting actions (slavery, stealing land from Native Americans, treating women as chattel) were as well.