They did not assume that. That was the price to get the money.
Essentially it was like loaning. Land could not be pawned away or used as collateral. There were laws that forbid the alienation of the feudal land. But the noble had no money himself, only what the serfs gave or what he got from the sale of the produce from his lands and his serfs. So they give the right to collect the tax.
Just occurred to me that I might have missed what you really meant and my explanation is unnecessary.
