'Get out of jail free cards'
I often hear people refer to a champions mobility or evasion spells as 'get out of jail free cards'. This doesn't really make any sense though. In the first place, such a statement seems to imply that if a champion, lets say fizz, didn't have his mobility spell then they would have no spell on that button. If you take away fizz's e then he only has two basic spells and an ultimate.
But this obviously isn't what would happen. If riot designed fizz to not have his current e or took it away in the future, they would with 100% certainty replace it with something else that was similar in power budget. The same is true of any other champion with a mobility spell or escape.
So clearly the spell isn't 'free' - there's an opportunity cost that goes into a champion having a mobility spell (or any spell really) on a fundamental level.
Even if you try and be pedantic and point at champions who don't spend mana and say that their mobility is 'free' in terms of resources you would be wrong. Every champion has cooldowns, so using a spell makes it unavailable for a period of time at the very least and champions are balanced around these sorts of windows of weakness. Katarina may be a cooldown only champion, but if she uses shunpo to commit to going in on someone then she is extremely vulnerable for a relatively long time if she doesn't manage to get a kill or assist when doing so, and this is also true of if she uses it to escape but her route is blocked off. With her mobility spell down she is just food for basically anyone. Shen, zed, kennen, and leesin use energy to power their spells, which while it is infinite in the long run, in the short term over using mobility spells will run them out of energy fast enough that they won't be able to use their spells even when they have the cooldowns.
Opportunity cost is the name of the game. Immobile champions tend to beat the crap out of mobile champions who come into their range and eat their damage rotations, the trade off is that a mobile champion can better pick when to go into and out of that range.
By the way, the whole idea of a 'get out of jail free card' implies the belief that you 'earned' a kill on someone or they 'deserved to die' for their positioning or decisions, but the reality is that clearly if they got away using their kit inbuilt escapes or flash then they were actually in a safer positioning than you thought and it was YOU who misread the situation or got more bloodthirsty than the situation merited.
Champions have different strengths and weaknesses, and any decent player is going to try and play the game on their terms by taking advantage of what their champion is good at. So don't get all mad that a leesin for example was 'caught' in your jungle and 'got away for free' because of his mobility when he 'deserved to die for being caught'. He just has more positioning options than less mobile champions that he has to exploit to bring out his champions full impact on the match. For the same reason leesin shouldn't be mad at a darius who flanks his escape path and catches lee without any escape or ult and blows him up, then walks away past 2 of lees teammates who came to help while shrugging off their damage by spin healing. Different champions have different options.
TLDR: Opportunity cost is a thing so no spells in any champion's kit are 'free'. Different champions have different positioning options that make sense for them based on their kits. You haven't 'earned' a kill on someone unless they died.