CM Punk and Brock Lesnar are different situations.
CM Punk came to the UFC with zero MMA experience. His first fight was a joke that was designed to sell PPV. Punk, if he had won, would have probably gotten another fight against an ACTUAL UFC fighter before getting a title shot. Don't forget that Mike Jackson was brought into the UFC SPECIFICALLY for that fight...he wasn't/isn't a rostered UFC fighter. You're right though, the UFC is a for-profit company, so they need to make fights that sell pay-per-views. But there is still at least a little respect for the belt.
Brock, despite his history with steroids, despite his losses (how many times has he actually fought in the past 8 years? 3, and the Hunt fight was still a win even if it became a NC) is still a former heavyweight champ. He has some legitimacy as a title contender, ESPECIALLY in the UFC's abysmal heavyweight division. CM Punk doesn't have any of that.
Does Jon Jones deserve a title fight immediately upon his return? He's tested positive for steroids twice, and has only fought one time since USADA started working with the UFC without popping hot. That one fight was against OSP, and he looked less than great. Jones has had numerous other issues outside of the sport. He's also a big draw. Does he deserve a title shot? Yes, because he still has a legitimate background in MMA and in the UFC.
There's more that goes into this debate than just "Oh this guy is a big name so this is a sham fight." Brock RAN the heavyweight division until he got sick/ran into Cain and The Reem (there's probably room to debate how much his illness affected those last couple of fights). Despite the steroids, he beat Mark Hunt in a hard-fought decision. The UFC heavyweight division isn't very good right now; Brock is a good way for the UFC to sell some PPVs using a big name with a legit MMA background.