I'm not a lawyer; this answer comes out of my (US-centric) understanding of basic licensing and copyright fundamentals. I'll address some basic US copyright law and case law in an attempt to shed some light on the topic, but don't take anything below as definitive (except the last two sentences -- if you want a tl;dr, read those).
Lamson's license is very bad, for a large number of reasons, and the author of Salmon was wise to move as far away from it as possible.
Some US-centric copyright and licensing review:
In the US, copyright offers monopoly rights over six particular actions:
(1) to reproduce the copyrighted work in copies or phonorecords;
(2) to prepare derivative works based upon the copyrighted work;
(5) in the case of literary, musical, dramatic, and choreographic works, pantomimes, and pictorial, graphic, or sculptural works, including the individual images of a motion picture or other audiovisual work, to display the copyrighted work publicly; and
[(3), (4), and (6) omitted, as they relate only to audio and/or performance works]
As you can see, "in the case of software, run the program code" is not included. This suggests that if you have received a copy of the software already, you can do whatever you like with it. Imagine an analogous case with a book: when you receive a book in a sale, copyright disallows you from copying it, but it has no say in how often you can open it and read it.
This point is heavily muddied, however, by software licensing. If you only use software under a license (i.e., it is "licensed, not sold"), you have not actually participated in a bona fide sale-transfer of a copy of copyrighted material -- you have been granted some limited, license-dependent rights over that copyrighted material. The ruling in Vernor v. Autodesk devises a test to determine if a particular transfer is a sale or a license:
First, we consider whether the copyright owner specifies that a user is granted a license. Second, we consider whether the copyright owner significantly restricts the user’s ability to transfer the software. Finally, we consider whether the copyright owner imposes notable use restrictions.
In the narrow case of Lamson's BSD-like license, it certainly passes the first test, but probably fails the other two tests. However, it's difficult to guess exactly how a court might rule here, as it is painfully obvious that the license was not composed with the intent of being legally rigorous. Let's assume (rightly or wrongly) that this BSD-like license is actually a "sale with restrictions" rather than an actual license. From there, we'll infer from that fact that right-to-use is granted naturally, unfettered. (This means that for other licenses with more rules, like copyleft or proprietary licenses, this may be a significantly less safe assumption.)
To review our assumptions thus far:
- Lamson's license is actually a sale with restrictions
- The sale with restrictions implies an unfettered right to use the software (this is perhaps a more dubious assumption, and I consider it first as true and then again as false, below)
Thus, if we assume that the preceding analysis is true: if you have a copy of the software, and your license is revoked, your only obvious responsibility is to immediately stop distributing the code. If you aren't distributing the code in the first place (e.g., you use it as part of a network service), you can continue using the software, as is your natural right as the recipient of a sale-transfer.
If we assume that the Lamson's license does impose enforceable restrictions on use (either because it passes the Vernor license test after all, or the sale-transfer does come with enforceable use restrictions), then a user would indeed need to stop using the software as well, or else be legally liable for violating the sale restrictions.
The user would need to comply immediately, but it seems extremely unlikely that the copyright holder could impose a retroactive revocation of the license. If the copyright holder informs a user that his license is revoked, the user might be held accountable for violations from that point onward. (To what degree a user might be held accountable for automated violations -- e.g., the license was revoked on Sunday, but the file server kept serving files to downloaders until it was shut down on Tuesday -- is left as an exercise for the courts.)
This license is also quite troubling because it requires private modifications to have their copyright assigned to the original Lamson copyright holder. It is totally unclear if those private modifications must be sent to the author, however. Again, this license was obviously not written by someone who gave (or was trained to give) significant legal consideration to the license.
Please don't write your own license.
If you must, consult a lawyer.