Like て当然（とうぜん）だ, the grammar point indicating that (A) is ‘natural’ or ‘obvious’, のは当然（とうぜん）だ works in a similar way, and highlights that it is ‘no wonder that (A)’, or ‘it is only natural that (A)’.
This construction will be seen following verbs and い-Adjectives, or な-Adjectives and nouns that are followed by な.

• 会社（かいしゃ）のお金（かね）を勝手（かって）に使（つか）ったなら、クビになるのは当然（とうぜん）だ。
  If you did use company money without permission, it’s only natural that you get fired.
• このタワーマンションは六本木（ろっぽんぎ）にあるので、家賃（やちん）が高（たか）いのは当然（とうぜん）です。
  This high-rise condo is located in Roppongi, it is only natural that the rent is expensive.
• おばあちゃんは毎朝（まいにち）１０キロも散歩（さんぽ）しているから元気（げんき）なのも当然（とうぜん）だ。
  My grandma walks 10 kilometers every morning, so it is no wonder she is so healthy.
• この1年間（ねんかん）全然（ぜんぜん）勉強（べんきょう）をしてこなかったから落第（らくだい）も当然（とうぜん）だ。
  I have not studied at all for the past year, so it's no wonder I flunked out.

As can be seen in several of the above examples, も may replace は, with only a slight change in nuance (も will place extra emphasis on the obviousness of the remark, according to the speaker).
Compared to て当然（とうぜん）だ, のは当然（とうぜん）だ is slightly stronger, and separates the previous statement from what the speaker considers ‘natural’. This is equivalent to the difference between ‘being happy is natural’ (slightly weaker), and ‘it’s natural that one would be happy’ (slightly stronger) in English.