[reaperducer]

It is absolutely a red flag. If you've committed to another company, then you have no business interviewing elsewhere.

If you don't take a job seriously at one company, why would I expect you to take a job seriously at my company?

If I offer you a job, how can I be sure you'll show up for it, and not just take the next offer that comes along? I can't. So you're no longer in consideration as soon as you tell me you're disloyal to your other company.

In some industries in America (especially media companies, but it's also common in high-end retail), if your boss hears that you're looking for another job, you're immediately fired. Sometimes it's even written into the contract, if you have one.

[matz1]

If you are serious then you better give a generous offer so that is unlikely for other company to give better offer.

A job is a business. The loyalty is goes only as far what the company or the employee can offer.

Likewise a company can fire employee anytime as soon as that employee is not needed anymore or better employee come along.