Where did you get your information? All hens from laying farms go into chicken soup or other chicken products. There are none that are “killed and buried”.
I suggest you visit Arkansas where many are processed. Simply stated, the large-scale processing plants are designed for 'Broilers', not laying hens. The plants are largely automated, and the old laying hens are too large to run through the process. Again, have you priced what whole chickens costs at the grocery store? Now, figure in the cost they pay, and then backpedal for shipping, refrigeration, inspection and handling. You are looking at around $0.58/lb for a whole bird.
If you get Netflix, there is a fascinating documentary called "Food Inc." that deals with this 'problem'. The cost of chicken meat is so low (~$0.58/lb) that hand-processing the meat at $16/hr labor costs (including taxes, insurance, Soc. Security, Welfare, and misc. benefits such as sick leave and vacation) - that it's simply easier and cheaper to gas and bury. Yes, it's a shame - but look at the economics of the situation. How many "free" hens must be processed per employee, with an employee cost of $16/hr - to be able to sell that meat for $0.58/lb? That is the wholesale price (give or take). Why is the cost so high? Well, for starters it's dangerous work, lots of sharp knives around, secondly it's not pleasant work at all - in fact, it's quite unpleasant. Lots of turn-around and re-training of employees.
Let's see, assuming no breaks, trained employees, each processing 27.58 birds per hour. This assumes ZERO transportation costs, ZERO cost per bird (hens paid themselves off during laying), ZERO inspection, ZERO loss of birds during processing and your plant uses No power, is perpeturally clean and maintained free, and you got the plant as a gift ... This is where we are.