It was NOT Sr's decision to make. Jack had already made his final wishes known to Lureen. I don't know how he got his hands on the ashes at all. Lureen must have made some arrangement, but if only she'd kept them all a little while longer,Ennis would have been able to carry out Jack's last wishes without his interference IMO. Lureen did'nt know if BB mountain really exsisted (so she says) until she talked to Ennis and it was she who requested that Ennis visit Jack's folks to retreive them right? I agree WW. Sr had to live with the fact that he denied his own son happiness, and in this sense he was "buried alive", This would work for me if I thought the man had a concience but if not at this Important milestone in his life, (the murder of his son for Christ sake), when, oh when did he plan on reedeming himself in the eyes of his long suffering wife?, his other close loved one and to his son's memory? He had the chance when Ennis came to him, but what did he do? well......we know.
Maybe, and thats a BIG maybe his insistence on having Jack's ashes there with him was his way of keeping a part of his son alive with him, I doubt this but it maybe a way to redeem himself in some form after all. The thought did cross my mind. Fleeting though it may be. My two cents.