The original Apache license was similar to the Berkeley license, but source code published under version 2 of the Apache license is subject to additional restrictions and cannot be included into OpenBSD. In particular, if you use code under the Apache 2 license, some of your rights will terminate if you claim in court that the code violates a patent.
A license can only be considered fully permissive if it allows use by anyone for all the future without giving up any of their rights. If there are conditions that might terminate any rights in the future, or if you have to give up a right that you would otherwise have, even if exercising that right could reasonably be regarded as morally objectionable, the code is not free.
In addition, the clause about the patent license is problematic because a patent license cannot be granted under Copyright law, but only under contract law, which drags the whole license into the domain of contract law. But while Copyright law is somewhat standardized by international agreements, contract law differs wildly among jurisdictions. So what the license means in different jurisdictions may vary and is hard to predict.
Not a dumb question at all -- that's actually fairly controversial.
Many licenses are offered as part of a contract. But in _most_ jurisdictions (albeit not the one I'm standing in, which is Scotland) in order to have a contract one must have both a meeting of the minds and consideration. Many Free licenses don't have either. So they rely on copyright law: you don't have to agree to the license terms. If all you're doing is stuff that's allowed under law, you're fine. If you want to do something beyond that, you need permission and the license is what supplies permissions.
This is also where the "viral" nature of GPL licenses has its controversy: if the license is a contract, then you've agreed specific actions to take in order to be able to derive from the source. So (while it's not likely) a court could compel specific performance of that action, forcing you to release your code under the GPL (viral!). If it's not a contract, then by not agreeing to the terms you may have breached copyright, but there's no mechanism for anyone to _force_ you to do anything except stop breaching copyright. So no possibility that you will be required to release your source.
So you may be better off thinking of the license as a thing that may be bought or sold using a contract, but is itself not a contract. Proprietary software will often be bought or sold like this: you're not buying _the software_, you're buying _a license to use the software_. Where Free software licenses are often (but not always!) available for free.
But some licenses start looking like they stretch beyond the realm of pure copyright licenses, and that's where the controversy picks up again.
How do shops work in those jurisdictions? Don't they usually rely on an ‘offer’ that is semi-implicitly accepted by a client? And which is afaik a form of contract? I thought that this is the mechanism that licenses use.
User andinus provided the official explanation given by the openbsd project.
On a more philosophical level, the more radical end of the BSD licensing champions such as openbsd are extremists, just like the FSF/GNU people on the other end of the spectrum. Both want a world where ultimately software copyright, not to mention software patents, doesn't exist, source is available for everything and you can use that source for any purpose you like etc. They just have different ways of working towards that goal. The GNU end came up with a legal trick to use copyright law to try to force people to open up their software (copyleft licensing), whereas the BSD radicals just wish the whole copyright crap would go away and thus use the simplest possible license they can come up with.
So the Apache-2.0 license, although in many ways a BSD-like license, is written by and for corporate lawyers, which grates the openbsd people to no end.
Uh, maybe I should have worded that differently. I didn't mean "extremist" as a value judgement. Just someone being on the extreme end of the spectrum "all software should be free" vs. "all software should be proprietary and patented".
Edit: And now I see that in my grandparent post I wrote that the BSD and GPL people are extremists on opposite sides of the spectrum, which doesn't really match the above. I don't know what I was thinking when I wrote that, what I wanted to write was that they are both on the same end of the spectrum.
