It is the logical result of legal paranoia. In the companies mind they don't want to get sued for royalties from an employee who claims that the big thing he implemented belonged to him due to it being developed prior to joining the company, so you get prior ownership. Similarly companies don't want folks leaving when an idea they are having is just getting interesting and starting their own company by regurgitating the ideas and claiming they own it, hence the post ownership.
As an employee you are a cog. You are not a person. You are part of the machine to make money for the shareholders (well, to line the CEO's pockets to be more honest). Your best interests are not on the top ten list of company concerns, unless it affects shareholder value in a big way.
When every company has about the same agreement you are stuck, and at this point all companies copy each other's HR/legal framework. Either start your own business, or sign on dotted line. Some are worse, and enforcement varies widely, but I have yet to work at a place without such boiler plate policy.
I am a hardware guy. My approach is spend a little spare time finding old expired patents and publications that are close enough to my idea to convince my management it is not an invention, but just solid engineering. Sometimes I stretch that and get away with it due to only semi-technical managers... I have no patents, and want to keep it that way. I've had a few ideas I'd like to keep in my toolbox, sadly none have had direct applicability, but I have preserved my option to due so.