I don't htink that's necessarily true. If they buy the bitcoins a month before paying the salaries, things should go well. Things will only go badly if they decide to wait until endo f month to pay via bitcoin, and the price tripples in said month...
Let's assume employee receives 1000 USD as salary each month.
Employer buy 2 BTC at price of 500 USD. That means the employer spent 1000 USD to acquire the total amount of BTC.
The pay day comes and the employee decides to receive in BTC.
Are the employer going to pay 2 BTC (which cost 1000 USD) for the employee? No. The employer is going to pay the amount equivalent in accordance with the current exchange rate due two reasons:
1. If the 2 BTC worth more than before (let's assume 1 BTC worth 600 USD), the employer is certainly not going to pay a salary higher than he promised.
2. If the 2 BTC worth less than before (let's assume 1 BTC worth 400 USD), the employee is certainly not going to accept a salary lesser than was promised.
So everything is fine until someone will have to pay for the loss. When that come in scene, I am certain that neither employer or employee are going to accept the loss.