In my entire career (as a professional software developer) I'm pretty sure every employer I ever worked for gained far more from my own personal development off the clock than they ever gained from any training they provided, not least because the main "training" most of them provided was simply a chance to use my skills and develop my experience. I therefore find the whole idea that an employer is somehow nurturing and developing someone in our profession and so should be entitled to everything that person ever does, even off the clock and even into the future, a bit bizarre.

Even if it weren't, employers aren't hiring machines, they're hiring human beings. If they've become so greedy that they have forgotten that, I see no reason our laws shouldn't compel them to treat their employees (and past employees) properly.