Probably because the cost of the PR nightmare is either not easily quantifiable, or simply an acceptable loss; wheras the potential "forced profits" by people having to buy the remaster are fairly easy to predict.
But that PR fallout hits Valve much harder than Ubisoft, because unlike Ubisoft Valve still has a reputation to lose. That reputation is the main source of the stranglehold Valve is holding the entirety of PC publishers in (except Blizzard). They might in fact consider the PR nightmare a positive side-effect of shutting down, leveling the playing field.
Because if they do, the next time they sell a remastered anything, people will think "I don't need to buy this because I'll get it anyway when the servers for the original version run out," costing them sales.
I can't believe they're not doing this. The remastered version is frequently on sale for just $20, and revoking access is clearly a PR nightmare.
If Ubisoft so dearly needs to shut down these servers, why not just gift existing owners the upgrade?