If by contractual obligations you mean "clauses that are most probably illegal but the average person will usually just shut up and pay rather that lawyer-up against a multibillion corporation", then yes I can see how many, me included, just opt for one of those cards whenever possible.
Maybe if companies didn’t make cancellation processes so actively hostile to users or throw in mysterious charges that continue accruing for all eternity, people wouldn’t be so uncomfortable using their normal card?
And don't forget that on top of exploitative practices, corporations frequently game the legal process too... like forcing customers to sign away their rights to sue in court, and all that.
Edit: one of many examples https://www.washingtonpost.com/business/2021/06/02/automatic...
No, it's not, an even if it was, that would create no obligation of a vendor to let you use it to establish an account. The effect of legal tender comes when making an attempt to pay an existing debt, not when trying to establish a relationship in which charges might be incurred in the future.
Either laws are way too old for the current times or were made worse by lobbying groups in favour those with might.
Things don't seem to be getting better (cookie annoyance being one example) unfortunately.