Is it permitted to look for a job while visiting the US under the Visa Waiver Program?

So there are really two questions here:

1. What do laws and regulations say about VWP visitors looking for work during their stay in the US?
2. Practically speaking, if I want to visit the US to look for work, should I apply for a B-1/B-2 visa instead of trying to use VWP?

To the first question, the answer is that the allowed activities on VWP are exactly the same as the allowed activities on a B-1/B-2 visa. However, one may not apply for an extension of stay, change of status, or adjustment of status while on VWP. (There are some exceptions to this rule.)

If you don't believe me on this, here are the legal sources.

The Immigration and Nationality Act (INA) §217(a) lists the requirements that are imposed on the visitor themselves. There are various requirements that don't apply to B-1/B-2 visitors (e.g. must be in possession of a round-trip ticket, must not have visited Iraq or Syria, and so on). However, the sole requirement on the allowed activities while in the US is:

(1) The alien is applying for admission during the program as a nonimmigrant visitor (described in section 1101(a)(15)(B) of this title) for a period not exceeding 90 days.

The Foreign Affairs Manual, while not binding, reflects the State Department's official position regarding the appropriate uses of various visas. Regarding VWP, it mentions here:

An alien who is a national of a participating VWP country does not require a visa, provided the alien:

(1) Is applying for admission as a nonimmigrant visitor, as described in INA 101(a)(15)(B);
(2) Seeks to enter the United States for a period not to exceed 90 days;
...

and:

Nature of VWP Travel:

(1) Maintenance of Status: An alien admitted to the United States under the VWP:

(a) Is admitted as a visitor for business or pleasure for a period not to exceed 90 days;
(b) May not engage in activities inconsistent with status as a visitor;
(c) Is not eligible for an extension of stay in the United States;
(d) Is not eligible for adjustment of status to that of a lawful permanent resident alien (other than as an immediate relative as defined under INA 201(b) or under the provisions of INA 245(i); and
(e) Is not eligible for change of nonimmigrant status.

The CBP Inspector's Field Manual may be more directly relevant to this situation since most ESTA applications are not manually reviewed by someone at the State Department but the determination of admissibility will be made at the port of entry. Regarding VWP, it says:

The Visa Waiver Program (VWP) permits nationals from designated countries (listed in 8 CFR 217.2(a)) to apply for admission to the United States for ninety (90) days or less as nonimmigrant visitors for business or pleasure without first obtaining a U.S. nonimmigrant visa (USNIV).

"Business or pleasure" is the same as a B-1/B-2 visa.

It also mentions:

An applicant for admission shall not be admitted under the VWP unless the alien convinces the examining immigration officer that he or she is clearly and beyond doubt entitled to be admitted and is not inadmissible under section 212 of the Act. The conditions for admission are specified in section 217 of the Act and 8 CFR 217. All VWP admissions are for 90 days unless the applicant’s passport is valid for a lesser period, in which case the period of admission would be until the expiration date of the passport for those countries on the 6-month list. In the cases of those countries not on the 6-month list, the applicants would not meet the documentary requirements in Chapter 15.7(b), Documentary Requirements, and would be inadmissible under the VWP.

This doesn't add anything beyond the statute and regulations which already specify that VWP applicants may engage in the same activities as B-1/B-2 applicants. The "clearly and beyond doubt" standard applies to all nonimmigrant applicants for admission.

In short, if you believe that B-1/B-2 visa holders are allowed to enter the US to look for work, you should believe the same about ESTA holders.

Regarding the second question, my opinion is that you should just try to enter under VWP. It's very unlikely that they will tell you "no, you have to apply for a B-1 visa for that" since they are supposed to know that the allowed activities are the same. If they do refuse you admission under the VWP, you are unlikely to be approved by ESTA in the future. However, that would also be the case if you were refused admission under a B-1 visa. So, to preemptively apply for a B-1/B-2 visa now would be pointless in my opinion. If you are reading this during the COVID-19 pandemic, trying to apply for a visa would needlessly inconvenience you because many consulates are not operating at full capacity anyway.