The majority of the film is set in 1987 California and the state had no motorcycle helmet law until 1st January 1992.
To evaluate helmet use in California before and after the introduction of an unrestricted helmet use law on January 1, 1992, observations of motorcycles and their riders were made at 60 locations in seven California counties, twice before ...
Bumblebee demonstrates a capacity to use radio signals in order to (re-learn) English when his vocal unit is damaged by Decepticon fire and subsequently replaced. Note that this would also strongly suggest that he's not speaking English, but rather his vocal unit is translating spoken Cybertronian into English.
Charlie and Memo stared at the radio, then ...
Because it's not the law.
Although many states have some laws requiring wearing of helmets on motorcyles / modes, this is not universally the case.
Motorcycle helmet laws vary widely among the states and have changed a lot in the past half a century. Currently (April 2019), 19 states and the District of Columbia have laws requiring all riders (operators ...
She's being sarcastic. His decision to turn into a car is clearly not helpful in the situation and nor are his self-pitying actions and feelings (a "shame spiral") after having trashed the place.
Stripped of the sarcastic tone, it becomes
Charlie: I'm unhappy. The fact that you've decided to feel sorry for yourself and turn into a car is obviously not ...
The intention for the film seems to have changed over time
Originally, the movie was conceived as a straight prequel to the 2007 Transformers film, showing Bumblebee's exploits on Earth during the Cold War. Remnants of this concept can be seen in Bumblebee's own prequel comic. However, this concept changed substantially during production, ...
According to: Helmets in the USA
States began to adopt laws on wearing helmets for bicycle riding in
Moped are technically classified as "Motorized bicycles".
The film Bumblebee is also set in 1987.
Hence, that law might not have been in effect in that area at the time.