At the moment, labor market is quite dynamic. Many companies want to hire more employees while some workers are fired. It is a curious paradox that companies usually expect the loyalty from candidates, but employees are not treated the same. I suppose that changing company staffs occasionally is an acceptable idea due to current economic circumstances for the following reasons.
The world trade is changing dramatically, so everyone and organizations have to adapt themselves to update new conditions. If someone can not adjust himself to the new environment, should be fired. In order to survive, companies need to follow the market needs. They usually find the way to work and innovate their system. This requires the flexibility of manpower in order to work under new companies' rules or regulations.
In some cases, employees can take advantages from the changes. Such requirements encourage the employees to improve themselves. Following the new methods of work, workers can learn a lot. Some workers also have more chance to train. Furthermore, changing human resource gives opportunities to those who are skillful. It means that skillful workers can be promoted to upper positions where they can display their abilities.
In addition, the workforce which companies need is not always the same. In some period, companies do not have enough contracts with their clients. It means that companies' profit is low; therefore such companies are in nature to force to cut the number of staff.
Taking all the fore mentioned argument into account, I believe that companies' changing employees is acceptable.