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* Preventing major disruption to studios' projects as key players are poached any time a film goes into production. You could say they should have separate contracts binding the employee to stay for the duration if that's the concern, but that's not really practical, especially in an at-will employment state, where said contracts will be viewed with heavy skepticism by all parties. The more efficient way to achieve a similar effect is to just agree at the corporate level that you won't engage in this practice. This can have the side effect of limiting wages, but I don't think it necessarily has to.
* "Poaching" can be variously defined; the contract may make it legal for a competitor to process an applicant, but not legal to actively pursue staff that have not expressed direct interest. In such arrangements, employee mobility would be considerably less hampered.
I think those are good intentions. The business guy is trying to do right by the business, which is the sum of all of his employees. One could logically conclude that if films are constantly hindered by bidding wars, making 90% of the process recruitment, and secret information is constantly wrongly disseminated by that high rate of turnover, that his business may struggle and jeopardize the livelihoods of everyone involved. Some people may not believe it, but employee dependence does weigh heavily on good people in high-powered corporate positions.
One could also argue that the bidding wars just have to continue until the price for an animator stabilizes and that the companies just need to suck it up. These highly-editorialized articles definitely seem to make that case, and claim that the law backs up that perspective. That's fine if so. But it doesn't make Ed Catmull or the executives at every other firm involved in these pacts evil moustache-twirlers that exist only to steal from the middle-class, and we shouldn't be so quick to throw our own under the bus.