Hydro dams typically store water uphill for later. If you use it now, it's not there to use later.

In the case of a 'stranded' dam, there isn't likely to be a reason to generate power either now or later, so you might as well run BTC generation. I suspect this has been the motivation to build an awful lot of 'stranded' hydro dams, though...

You can store water in the short term. The long term view is simpler. The storage will eventually fill up if you're not using all the power, and at that point you either find a use or you dump the excess water and cause just as much waste heat.