It all depends on the licence. Even if you buy something on physical media you may not technically own it. If something has a FOSS licence MIT, BSD, GPL, etc Then yes you do own your copy and no one can change that.
I may only have a license to view the contents of a dvd, but at least I'll always be able to view it as long as it's in my possession and I have a dvd player.
Content you can only access remotely via someone else systems (or requiring remote authorization via there systems) can be taken away at anytime regardless of the terms of your license, even supposedly "indefinite/permanent/lifetime" licences.
Both of these items use the same term 'purchase'. This term used to refer to the first situation only, but now it covers both.
FOSS licenses are distribution licenses, not EULAs. You have the right to own and use software you acquire even without agreeing to them; they only "kick in" when you decide to do something that would otherwise violate copyright law.
I liked the explicit way version 2 of the GPL explained it:
Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program).
Version 3 says the same, but less clearly (note that "affirms" is entirely different from "grants"):
This License explicitly affirms your unlimited permission to run the unmodified Program.
EULAs presume to "grant" you something you already have due to the First Sale Doctrine (namely, the right to use your property) and are therefore complete bunk as they lack "consideration." If you believe EULAs are somehow valid just because the copyright cartel's shysters say so, you need to learn to quit taking advice from the enemy!