To be fair and non-reactionary: I have not read the details of this proposal. My understanding is that the tax is a 1% tax on wealth up to $1B and 1.5% for everything over $1B anyone who relocates out of the state. I presume this means that it would apply to wealth gained during one's time in California.
Forgetting the knee-jerk defense of this stupidity, how could this possibly be Constitutional? Are there some details I haven't read where this is a recoup of benefits given to people/businesses that the state feels they should be paid back for with continued income/property tax revenue? Sort of like how you'd owe back a part of the signing bonus if you left before a contract ended?
I don't see how this could be legal otherwise. Someone who knows better should educate me.