That's Joel's whole point, almost every single company had these restrictions written into your contract. And it's not possible to know if they're "the nice ones" who won't sue you ahead of time.
I may have just drunk too much HN kool-aid, but software devs seem to be criminally under-valued and often exploited (see Crytek thread on front page, for example). Then there's guys like Bezos who, after underpaying and exploiting you, will replace you with a machine-learning algo the second it is technically feasible and a net positive to the company bottom line.
I've also noticed some interesting features of the software development labour market: quite a lot of the work is creative in nature, you produce non-rivalrous products (i.e. my consumption of 'software x' does not block someone else's consumption), and the workforce is supposedly peppered with unusually talented individuals who produce 50-100x the value that the average worker does.
There are two other industries that have similar features: traditional screen entertainment (TV & Movie), and professional sports leagues. In both of these industries, the content producing workers (baseball players, actors) are invariably a member of an industry guild or union, and operate more like independent contractors than employees...
Food for thought.
A tangent, but http://www.telegraph.co.uk/finance/newsbysector/retailandcon...
There's a Primark on every high street in the UK. Someone you've seen today is probably wearing an item of clothing made in those conditions.
Don't work for companies that are dicks. If there's crap about them wanting to own you or your thoughts, don't sign it and get a different job elsewhere.
Remember when little kids would die in factories doing dangerous jobs? No? Me neither. Know why? Because at some point people had enough, went on strike, attitudes and laws changed. If you live in an area where your employer can make your life miserable for no apparent reason, or where your employer can own you outside of work, then you need to help put a stop to that madness.