I think the logic (pun not intended) behind it was "What if G1 Shockwave scanned a beast altmode but kept his normal robot mode just like in the G1 cartoon's pilot episode?"
In other words, the same kind of logic that the entire Siege line was based around: "Familiar robot mode look, but different altmode." Sure, that's not as fun as having the new altmode alter the robot mode in some way like in post-1980s series, but this team really seems committed to sticking to that G1 cartoon logic at all costs, for better or for worse.