て当然(とうぜん)だ is a commonly used expression which indicates that ‘(A) is natural’, or ‘(A) is a matter of course’. This is a combination of the て-form of either verbs of い-Adjectives (or で when used with nouns), the noun 当然(とうぜん) ‘naturally’, and だ.
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あんな生意気(なまいき)なやつは嫌(きら)われて当然(とうぜん)だ。
It is only natural that a cocky person like that is hated.
このスープは俺(おれ)が作(つく)ったものだから、美味(おい)しくてて当然(とうぜん)だ。
I am the one that made this soup so, of course it is delicious.
このビルは先月(せんげつ)建(た)てられたばかりだから、綺麗(きれい)で当然(とうぜん)だ。
This building was only built last month, so of course it is beautiful.
て当然(とうぜん)だ is a fairly strong phrase, and will often be used when pointing out things that the speaker considers to be obvious facts.