Many livestock farms keep their cattle "indoors" for 6-8 months of each year to avoid poaching the ground and for ease of feeding and management during the winter. Dairy farms bring their cattle into the milking sheds twice a day x 365/366. As I recall TB is spread by close confinement with infected members of the species. Maybe badgers are "one" of the vectors for spreading the disease to cattle. But maybe we're keeping too many cattle in close confinement for too long to manage their health properly. Maybe it's fundamentally a cattle-management problem? So we need a whipping boy. And tonight, Brock, you've won the prize...
(NB before any flames I would like to point out that I live in a farming community and used to keep cattle and sheep...which lived outdoors all year round.)
You're right, close contact is not the cause, it's the spread.
The cause is more likely to be the condition of the sheds in which they're kept - a dark damp shed is a perfect environment for the bacteria to grow.