OSNews

The legal doctrine is called "equitable estoppel". If you unilaterally promise something, you cannot then unilaterally withdraw that promise if, in doing so, you would cause harm to anyone who had relied upon that promise.

The classic case is this: you own a property in London. It is war-time, and you are finding it difficult to let the building. So you promise a potential lessee that you'll charge much-reduced rent for the duration of the war. The potential lessee agrees, and takes a lease. The war ends; you now say "you owe me the normal rent for the war years, which is much higher than the rent I actually charged you. And the war is over so you can afford to pay".

The English High Court, in 1947, basically said, "Bugger Off: you promise something, you can't go back on that promise if it causes detriment to someone who relied on your promise".

Microsoft promises not to sue in various circumstances; they cannot subsequently unilaterally withdraw that promise and hope to have their "rights" enforced.

And it's irrelevant if it's Microsoft or someone who has subsequently bought their patents or their patent right: Microsoft have renounced their patent rights in these specific circumstances. No-one may subsequently seek to enforce those rights, to the detriment of others, without bumping into the equitble estoppel brick wall.