It seems like they're ok with the poaching risk. I would assume most of their employees are happy enough, or they'd leave. I'm not sure having the salary posted is a big change from that perspective.
People don't work that way. There is quite a bit of discontent or discomfort people will put up with until they are sufficiently motivated to take action. The number of people who are underpaid by 30% and unwilling to look for another job is far greater than the number who would accept that offer once its in front of them.
