I think the people saying this is your fault are frankly wrong, ignore them. On the other hand, going in all guns blazing to give the security a bollocking is not going to help you.
Your bike could have been nicked. It wasn’t, so count your blessings.
Start a productive dialog with security about why you’re parking where you are, what can be done to resolve the situation and also mention that you’re not happy with how your bike was left unlocked. If you’re liable to explode if you go in there in person, do it on paper instead, though the best results are likely to come from having a friendly chat.
If you want to be angry about something there are plenty of more worthwhile things to be angry about.