Many lawsuits for harassment, retaliation, and wrongful termination by employees have gone to arbitration and their legal rights taken away.
Offer letters state in the event of employee disputes with the company it needs to go into private mandatory arbitration.
If you live in CA you have up to 30 days after signing to opt out. Otherwise, if you are harassed or retaliated against you can't sue in a court of law but end up in a kangaroo court outside of the legal jurisdiction.
Splunk has had many employment lawsuits moved to arbitration because employees didn't opt out because they think life is lollipops and unicorns until they get harassed at Splunk.
ALWAYS OPT OUT OF MANDATORY ARBITRATION BY ASKING THE HR PERSON BEFORE YOU ACCEPT AN OFFER!!!
If you've already signed the mandatory arbitration agreement you should think twice before you complain to HR or other staff because you'll get r@ped.
I agree you should always ask the HR person that sent you the offer letter the option to opt out and the timeline and procedure to do so.
God forbid you will be harassed but it's very common at Splunk and other companies and when it happens you have to protect yourselves from them harming you.
Splunk is a scummy company indeed.