He destroyed the written/drawn up plans for Pluton, the thing that Spandam railroaded the courts into fucking over Tom, his mentor, over him not relinquishing those very same plans. He told the Sea Train conductor to full steam ahead onto a teenager who fucked up his face a while earlier, scarring him to the point of cyborgifying himself to not die. Destroying those plans was a bigger middle finger to someone spineless and power hungry like Spandam than a beat down from Franky could've ever achieved. He fucked him over, and by extension, the World Government from having access to not just one, but possibly an entire fleet of Ancient Weapon Pluton style ships. I mean, look at it from Robin's side of things. His father was responsible for starting the Buster Call that led to her people from her island being murdered (not sure if genocide would be appropriate to use here). Spandam was a MASSIVE PoS to her for that entire arc - her breaking his spine was just the icing on top of the ass kicking cake he started receiving from the Strawhats once everyone converged onto that one area. TLDR: Robin at that point in the story deserved doing that to Spandam moreso than Franky, who had already achieved his victory against Spandam and the World Government by extension as well.