The thing is, if you've read a patent, then you (and your employer) now "know of" the patent. If you're later found to infringe the patent, the patent owner can ask for triple damages, since it was wilful infringement. But if you never read it, they can only ask for damages.

Given how many patents are vaguely worded, and how, um, interesting the decisions in patent cases sometimes come out, many companies decide that it isn't worth the risk.

Note well: IANAL. I've just been in companies that had this practice (don't read patents), and I remember why.

Yes, I understand the logic of it, and I have heard of companies that include this requirement. I just doubt that this is anything like a standard industry practice. I've been in the industry for a very long time, and have never once encountered such a restriction, even at very large corporations. I would think if this were close to being a standard practice, I would have encountered it at least once.