Tax
This practice has been going on for many years with international companies who send employees abroad for specific training, for periods ranging from six months to a couple of years.
Homer has put it excellently, so i shall not try to enhance what he wrote.
The tax equalisation calculations are usually done by a Big5 accountancy firm who are well experienced in tax equalisation for both expats and impats. The theory is that the employee should be no better or worse off from a tax standpoint, and that may involve surrendering a tax rebate from the tax authorities in either country. If the net result is that the employee is worse off, then the employee has, of course, a right to object. It would be very surprising if the employer did not deal appropriately with that objection.
The incentive package usually includes a consultation with the aforementioned accountancy firm to enable the employee to review and approve the tax calculations. The employee also has the option to have the calculations checked by an independent tax expert, but that rearly happens, because most employees are usually satisfied after the consultation.
This practice is not illegal or sinister, it is normal business practice and it should not be assumed that the employers are out to screw the employees.
(Before anyone says it, Big5 firms are not perfect and are not, in this case, independent, but they are pretty good at this stuff).