I'm just speculating:
A substantial part of LA's personal fortune is based on his 'name'. However, after his 'retirement' he probably noticed that his 'name' lost a lot value. So, to increase the value of the LA brand, he came out of 'retirement'.
The above argument can of course also be used in respect of his cancer campaign. While LA was king of the TdF, the LA cancer campaign went from strength to strength. When he retired the value of the name waned and support for the LA cancer foundation decreased.
Others might argue that if the value of his 'name' increases again after his comeback, the primary result would be an increase again in his wealth, prosperity (increase in appearance fees, advertising income etc) bit also his stature, i.e. being 'the Boss' again. The secondary result, increase in awareness of his cancer foundation, might then just be an excuse for the pursuit of the abovementioned primary result.
We can only deduct from his actions as to what his real goals were.
LA fans might argue that he already is rich enough and therefore we must believe him when he said that he came back for the cancer issue. This would, of course, ignore the importance, for LA, of his stature (fame).
The so-called 'haters' on the other hand, argue that his conduct during the TdF (towards Contador that stands as GT cyclist head and shoulders above the rest of the peloton) clearly illustrates that LA was not in it for cancer, but personal reasons.
When I weigh such arguments against each other, I must agree that if the overrding objective was for him to ride for cancer, he would have concentrated on advancing such cause and supported the best rider in the team(including moral support).