I'm going to put on my fan glasses now and explain why the Cobb angel was OK but the Johnson one wasn't.
Cobb was running a pattern in the endzone that caused him to go to the ground while making the catch. You can CLEARLY see that as soon as he completed the catch he was attempting to get to his feet with the intention of starting his Lambeau leap. A celebration which CLEARLY should be the only TD celebration allowed in the NFL but that's a discussion for a different day. The slippery surface caused him to lose his footing again and he slid onto his back and his momentum CLEARY caused his arms and legs to naturally begin the motions of the snow angel and, in the heat of the moment he just finished a few more sweeps of his arms and legs. It was CLEARLY not an intended celebration. In fact I'm not even sure Cobb had any control over the motion at all. It is possible that the entire snow angel event was the result of his body's natural momentum which Randal had no control over. His team mates could CLEARLY be seen standing over him attempting to help him to his feet and in no way were encouraging him at all in his attempt, if in fact there was conscious attempt, at making a snow angel. You can see that as soon as Cobb regained control of his body he immediately jumped to his feet and ran off to do his aforementioned only acceptable celebration, the Lambeau leap. it was CLEARLY a spontaneous act if, like I said, it was a conscious act at all.
Whereas the Johnson snow angel CLEARLY went down like this. The morning of the game Dontae Johnson found out that the gametime weather forecast called for snow so he immediately took out his cell phone and googled "snow angel." Upon seeing how easy they were to do he immediately set upon a plan to make a snow angel should he by chance score a touchdown. He asked his team mates and they thought it was a wonderful idea and would be the perfect opportunity to rub salt in the Bears wounds as well as all the fans in Chicago and the NFL as a whole, with Roger Goodell in particular, and indeed the world as we know it. They immediately sat down to figure out a gameplan that would give them best opportunity to carry out their plan. They got together with the coaches and they all agreed that the best opportunity would come on a blocked punt so head coach Chip Kelly tasked special teams coordinator Derius Swinton with the job of scheming to get it done. When the 49ers punt return team was taking the field prior to the ill fated punt Swinton was seen pulling Johnson to the side and while no one is privy to the exact conversation that went on a lip reader sitting in the 5th row immediately behind the 49ers bench, who came forward on the condition of anonymity, has stated in a sworn affidavit that what Swinton said was "this is your chance Dontae, go out there and make this pathetic shit filled season mean something for all of us. It's all on you. Make us proud" I suppose they could have been talking about something else but C'mon man what else could it have been. When the punt was blocked and Johnson recovered it and started running towards the goal line several of his team mates can be CLEARLY heard yelling "don't forget the snow angel Dontae" and "Here comes the angel man" On the sidelines Swinton was heard telling Kelly "He's going to do it, he's going to make the snow angel" and Kelly replied "I can't wait to see old Roger's face when he sees this shit. It's really going to hit the fan now. Fuck him if he can't take a joke" Once Johnson crossed the goal line his team mates surrounded him and urged him on with chants of "Angel. Angel. Angel" and with that Dontae Johnson proceeded to do just that. If that is not a pre-planned team celebration I don't know what is.
Removing my fan glasses now