You pay them lower but fair salary while you are training them and then pay them a fair salary when they are trained. If you train them, you've increased their value and you're expected to pay for that additional value even if you are the one that did the training. Training an employee is a capital investment with a much greater cost of maintenance, you just need to factor that in when hiring.
Just because you paid for the training, does not entitle you to keep paying someone in perpetuity at their salary before being trained.
Basically the thinking is this: Cost of training * expected return on equity = Amount I should be able to extract out of the trained employee forever.
Selecting arbitrary numbers to keep things simple, if it costs $1000 to train the employee, and you want a 10% ROE then you expect to earn $100 per year more on that employee, forever. If the employee leaves they take that $100 per month potential away.
This leaves a few paths for employers: 1) Pay less until you have a return of capital, 2) Dont up the salary after the "asset" is upgraded, allowing you to get a much ROE as possible. 3) Dont hire untrained people, so you can capture ROI from others' training.
All of the above ignores the opportunity cost of not hiring. I presume for most profitable and busy companies, the lost opportunity is greater than the cost of training.
Just because you paid for the training, does not entitle you to keep paying someone in perpetuity at their salary before being trained.