1997 Mazda Protege

I just had a mechanic (ASE certified) change the CV joints as they were clicking on turn in. On driving home, noticed the car very bouncy. Took the car back the next day. Now the mechanic says the front struts are shot/not working. He says that they may have been damaged or let the gas escape when the wheels were allowed to hang down when the car was up on the 'brake jack'. (This jack goes under the body of the car, and allows the wheels to hang freely)
The mechanic says the struts can become damaged if they are overextended while the CV joints are being replaced. If he knew this, why didn't he support the weight of the axle, or make a hanger to prevent the strut from extending too far? Sure, the struts may have been worn, but they were at least working. Now they are completely NOT working, and the tires are taking up the job of absorbing the bumps.
My question is: Is the strut problem because the mechanic wasn't carefull enough? Or is this problem something that couldn't have been prevented and therefore it is a risk I am now stuck paying for new struts?