I'm beginning to think none of you have ever worked in companies that actually produce IP.
First off - that's not what patents are for. Patents protect publicly known information - not trade secrets. And they don't do it very well. There are very few things you can defend with a patent.
What companies are concerned with is 'trade secrets' and 'know how' - and it's a serious thing.
"Then the burden of proof should lie with the previous company, there shouldn't be any possibility of preemptive clause within the former contract."
If you work at DWAVE for 5 years and learn everything you know about Quantum Computing there - and go to Google to work on Quantum Computing - then there is basically a 100% chance that you are passing on 'know how' and 'trade secrets' - otherwise - Google would not hire you.
Your comments about customers etc. are irrelevant with respect to IP issues.
If you work for ABC corp and they make a 'sales automation tool' - and you go off and build your own 'sales automation tool' - then fine. It's not about IP or trade secrets. You can do as you like.
But for other types of things, no way. You work at Nuance on Speech recognition - then you leave and build a nearly identical speech recognition system? That's probably IP theft.
There would not be any technology if IP could not be protected in some reasonable way.
I'm not suggesting that many of these contracts are crazy - and that there can be absurd limitations and weird corner cases - of course it's very tricky - but companies deserve the right to protect their IP in some manner.