Then again, our heroes got the 2,000 figure from their scans. Those scans failed to reveal the presence of Warbirds...
If the Vulcan ships were there primarily as noisemakers to mask the cloaked armada, then the Romulans would have had no good reason not to put some of their troops aboard the decoys, too. That's 2,000 more than they'd otherwise have been able to carry! The total strength could still be something like 75,000 - better than the 73,000 that the Warbirds alone could have hauled, at no extra cost.
So even if the plan was a simple invasion without the "they wanted those 2,000 dead" conspiracy theory, we should assume we didn't see the whole truth. How could we, in a secret plot involving invisibility devices? If those 2,000 troops really were a decisive force all by themselves, killing them would have been idiotic behavior from the Romulans. If they were mere icing on the cake, their deaths could be argued to be a lesser evil than the embarrassment of having them be temporarily detained; but not otherwise.