Romulan arrogance and overconfidence. I'm sure Vreenak believed that Starfleet was too "cowardly" to do anything so Rommie-like as to murder him.The bigger question was why Vreenak revealed he had seen through the forgery.
Right because no Starfleet officer would ever sanction murder. A senator like Vreenak would have been long dead had he been so naive. Though...he did end up dead.
I think Garak would have killed him even if Vreenak didn't spill to Sisko.
Vreenak had no way of knowing that this particular Starfleet captain had such a cozy relationship with a very ruthless ex-Obisidian Order agent that he'd overlook murder (and he very nearly didn't overlook it). That does not fit with Starfleet propaganda in the least.
You're suggesting Starfleet's full of wusses for not condoning murder, but thank Cardassians have no morals?
Not to you directly, but lets remember that human and alien cultures alike are not monolithic. And Trek's shown the similarities between individuals of different species are greater than the differences between species - so a good human and a good Cardassian are a lot more similar than a good human and a bad human.