why does he get a pass while Janeway is condemned?
Firstly, the episodes are doing completely different things, and have completely different presentations. "The Enemy Within" uses the transporter malfunction to examin the duality of man, and doesn't address the ethics of the situation in any way. That's going to inform the viewers' reactions, just as bringing Janeway's decisions regarding Tuvix to the forefront of that episode informs the viewers' reactions of that.
Secondly, Tuvix himself would have agreed with Spock - at least, at first. He was initially an active participant in trying to find a way to undo the situation. Over time, though, he changed...and so did Janeway and the Voyager crew. Tuvix is given a name. He's given a job. Janeway calls him an officer and an advisor.
In short, Janeway granted Tuvix personhood...and then unilaterally stripped it away.