As part of my job, I help to negotiate contracts when my company is purchasing software (or hardware with embedded software). For these significant business relationships, we typically negotiate a Master Agreement, which govern all aspects of the relationship, including Terms of Purchase, Product Support, and/or Service Contract terms and more.
Despite spending months working through dozens of pages of terms; every software supplier also insists upon some version of this one-line statement, "If any of our current or future software, used by Customer, contain a EULA, it is hereby incorporated by reference."
In my experience, any time I've objected to the Supplier leaving a perpetual backdoor to arbitrarily insert or modify terms, I've been met with great shock and resistance. The attitude seems to be that the EULA is a foundational legal right that software makers have and it is beyond negotiation.
I don't have formal legal training or a deep background in contracts, so I'm seeking useful context or knowledge. Is there a historical or legal reason for software vendors to think that embedded license agreements should be entitled to ride along unimpeded and unseen on a parallel path through the contractual relationship?
Are there other content areas in, deals among relative equals, where it's common for one party to expect open-ended, unilateral power to remake terms; which might inform how I approach these negotiations?
UPDATE: Please note that I'm only asking about situations where the seller is the developer, we will be one of their largest customers, and they typically have hundreds of total customers.