I think it depends. When you entered the US you got asked the reason for your visit, and you probably said "tourist." If you had said "I'm going to look for and apply for jobs" (or "find someone to marry") you would almost certainly have got turned away. But if you come to the US (or pretty much any country) as a tourist and find a job by accident (or fall in love and get married) there's nothing illegal about that. It comes down to your intent, which is hard to prove.

When you apply for a job while in the US but you don't have a visa that allows work or business, you put the potential employer in a bad position. If they follow up and interview you and proceed with hiring you, they may be party to immigration fraud (more likely they will worry about that, I think actual legal action unlikely). And they will have other legitimate concerns: they will get started on the complex and expensive visa sponsorship process only to have you fail to get the visa (if USCIS asks questions about when you applied for the job), or that you will apply for other jobs because you mainly want the visa, or if you will work for them just long enough to get a visa. Most US companies don't have a lot of experience hiring foreigners so they might not want to take any chances. The companies that do hire a lot of foreigners can pick and choose from a large pool of candidates who want to work in the US.