They're not doing on-the-spot signature analysis. You're just harming your own chances of getting a dispute resolved in your favor if you ever need to make one.
When you have a documented history of legit signatures on file, signature difference is one of the factors that get considered in the event you file a dispute.
As of this month (April 2018), the big four card networks have stopped requiring (USA) merchants to collect signatures if chip is used. They had already waived this documentation step for the ~80% of transactions below $50.
I work in the credit card processing industry and can't even begin to comprehend how a fake signature would help.
It wasn't a valid signature. Even when you attempt to make a legitimate signature on a screen it comes out incredibly poorly. The only legitimaticy is that "someone signed some form of letters/made a drawing of a penis". I can't see why that would ever prevent chargebacks
Signatures do absolutely nothing in that case so why bother?