You were trying to affect how Six Flags received money and comparing them to other parks. Most parks dont really run things the same way. Most likely Six flags received many other complaints and just responded fully to a couple of them, but didnt want to deal with many others so..."Thanks for the E-mail - we will definitely look into it" that was there answer to a COMPLAINT. May have took it into consideration, but less likely to use it.
Most people would like to respond to a compliment with a full response than a complaint. And for Sea World the knew you would come back if they found your dad's keys or else you would find that park as insensitive. I lost stuff at parks and didn't get them back, but I thought about the park no differently.
This is your opinion and you just it was MINOR so who cares! The only thing that matters is that you went there and enjoyed your stay there.
Then write them a compliment about the park rather than saying you didn't. And don't say that " I never received good customer service". I went their and got great service, people who say they didn't are usually asking to much.