Rather than paraphrase the good explanation that I read on another site. I'll just copy and paste the explanation here:
...Sat is the past simple and past participle of the verb "to sit", which implies an action that only the subject can do on itself, nobody else but the subject of the sentence. So, you sat a customer on table 40, for example, is totally wrong, as one only sits oneself! However, I seated the customer, as you said, is right, because you are using the verb "to seat" as a transtive verb to indicate that you are the one who actually seated somebody else (took somebody to their seat). Sit can only be used against yourself, you sit on a chair, for instance. You simply cannot "sit somebody on a chair"! Therefore, in past form, they cannot be "sat" by you.
Basically seated and sat are from different verbs: the first one is "to sit" and the other one is "to seat"; just think of them like this: to sit oneself; to be seated.
I hate it when I hear people say "I was sat there"... it's ridiculous! You either say "I sat there", or you say "I was seated there". It's actually very simple when you think about it, people tend to hear these errors and they get stuck in their brains and become normal discourse.. let's protect the English language!!
This explanation is founded on the "fact" that "sit" is always an intransitive verb, i.e., that one can sit, but cannot sit someone/something else. A lot of dictionaries disagree. For example, Merriam-Webster online
(defs. 1 and 4 under transitive verb, as I quoted before), Oxford English Dictionary
(def. 23 with examples as far back as the 15th century, as well as from notable authors like Dickens), dictionary.com
(def. 16), Cambridge Dictionaries Online
(transitive definition marked by [T]). All of these list transitive usages of the verb "sit," and all include definitions that would apply to seating a customer in a restaurant. You can dislike the usage all you like, but that doesn't make it wrong.