First off, you don't own the app; you only have a license to use it. Secondly, if you read the legalese it's not Microsoft who kills the apps; that's totally on the app publisher. The license exists as long as the publisher publishes the app. Once they pull it, the license ends. Microsoft simply acts as a... distribution center of sorts, with their own license to use and distribute from the publisher.
Reading the terms in full [http://www.windowsphone.com/en-us/store/terms-of-service] they are actually rather vague on pulling applications from the marketplace, consumer license duration and type, recourse when an application is terminated, difference between a paid and free applications, etc...; all that the terms state is that: "If Microsoft disables the ability to use the applications on your devices pursuant to your agreement with Microsoft, any associated license rights will terminate."
But the key part when reading any overall vague statements like that is: "Unless applicable law gives you more rights despite this limitation" so in other words the laws you need to consider are the consumer protection laws and contract laws of your country.
And as I mentioned above "a license that can be revoked at any time for any reason whatsoever"; is extremely unlikely to stand up to legal scrutiny, as far as UK (and most likely EU, but would have to check the relevant Directives) consumer protection legislation goes. At the very least the consumer should be entitled to a refund in cases of paid applications being terminated, because the license terms do not expressly stipulate any limits to the license duration, except that it can be revoked by Microsoft at will, which most likely would fall under the designation of an unconscionable term.
On the other hand countries like the US where the consumer pretty much gets what is in the contract and where the consumer is not as protected, it might be a slightly different story, however I do not recall any consumer protection specific legislation from when I was studying NY state law few years ago.
[disclaimer: the above is not legal advice, just opinion and facts purely for informational purposes]
The bottom line is pulling applications from the marketplace without having a system in place for past users to re-install them is just a bad business practice and it does nothing to build consumer trust in the WP marketplace. Nobody wants a store where bought software disappears without warning, causing problems for people who depended and enjoyed it.
If Microsoft wants to build consumer trust in its WP and Windows 8 stores, this is not the way to do it.