I think it's because they didn't really know what they were facing and probably thought they could either disable the thing or defeat it.
Do you understand the magnitudes we’re discussing?
Using the DE’s 2 AU diameter, the size ratio between the V’ger cloud and the Klingon battle cruisers is about the same as the ratio between the planet and mini-cruisers about a centimeter in length. If the target is the size of an adult human, the attackers would be about the size of a proton. How could they possibly entertain thoughts of disabling or defeating that?
Well obviously the Klingon ship didn't back away. They kept going, so that got me thinking in these terms. And they did fire some shots at the thing... so...
Yes, I do understand the magnitudes you're discussing.