Not necessarily, many large companies are willing to guide and help an employee determine if something is okay or not to work on. You shouldn't be discouraged for posing the question, and if you are, the employer is not being rational.
The only case where it might even be a question is if you're directly taking what you are working on during your day job and putting it into a side project, aka stealing their IP.
Other than that, no, your time is your time and unless they are paying you then they have no say as to what you work on no more than they have a say in what you eat for dinner.
I disagree with your first sentence. Say that you work for Amazon, and you start an e-commerce side project that directly impacts the growth of e-commerce outside of the Amazon marketplace. It may be a negligible share of the market, but you are still negatively impacting Amazon's sales. There is a reason that they put conflict of interest in the fine print, and posing the question helps bring more context.
Doesn't always work like that. The non-compete and conflict of interest clauses are narrower than that. In case of Amazon, they sell pretty much everything, to everyone, so you might have a conflict of interest. But if you start an e-commerce store that sell something very specific to very specific market segment than you may not have a conflict. For example, B2B marketplace for oil supply chain industry (Know a guy who does something similar).
Same thing with consulting, a friend works for a consulting company that closes 1M+ projects. He on side make websites for small businesses. Everyone knows at his company, no issues so far.
> but you are still negatively impacting Amazon's sales
It is not the responsibility of a single employee. Even more in a market so ubiquitous as "ecommerce".
The same way, Amazon (and other tech companies) impacts negatively the careers of its employees when it change platforms, frameworks, technologies or do a layoff.
Why should a company hire someone who is potentially going to impact their own revenue? And in rarer cases, a side project that actually generates enough revenue to where the amount of money they are paying you is less than the share you are taking from their revenue.
To your second point, sure, employers hold more power. What is wrong with that? If you don't like it, you can always work somewhere else.
> Why should a company hire someone who is potentially going to impact their own revenue?
Because every software engineer can impact their own revenue the same way.
> To your second point, sure, employers hold more power. What is wrong with that? If you don't like it, you can always work somewhere else.
Don't liking it, I'm supporting my peers in HN that it is not unethical to have a side project and use it as a leverage to not be subject to the humor of your boss. It is also good for the company, as desperate people aren't creative.