The case was raised by an individual plaintiff, but in general vegans have a variety of different reasons not to want to encourage farming of animals.

Some of those include discomfort with eating animals and their byproducts, as you mention -- for others, it's based on a desire to encourage more sustainable environmentally-friendly food options.

For some subset of the spectrum mentioned, cross-contamination is an important issue - and for the rest, it may be acceptable under some circumstances, but there's no downside to removal of contamination, so they'd likely support it given the choice.