Technically the swerving itself is not necessarily illegal, but it makes you liable for anything that could have been avoided by not swerving. If you swerve on a free and open road and don't cause any damage or injury to anything or anyone (including yourself and your car), it's not like you will still be dragged to court just because you didn't want to unnecessarily run over someone's pet.

It usually means if you swerve and cause an accident, it's your fault and you can't blame it on the animal jumping out in front of you.

Probably because it would be impossible to prove that there was an animal there at that time.

Many cars have dashcams now, especially Teslas.

But definitely the law could, and probably is, out of touch.