It’s always a scam because money is fungible and unless the underlying asset itself is held by a separate entity, you’re always at the whim of the party in power.
Not OP, but tying some new revenue source to some existing expenditure very often is. A classic example in the US would be lottery and casino revenues. Bills or referrenda legalising these (fairly recent, largely since the 1980s) were generally sold to the public as a source of revenue for public education. In the case of California, those services had been paid for through state and local property taxes which were capped (or kneecapped, depending on your point of view) through the 1978 referrendum Proposition 13, see: <https://en.wikipedia.org/wiki/1978_California_Proposition_13>.
In the case of lottery / casino revenues, the initial messaging was that these would provide additional funds for education. In time, general funds allocations for education have declined, justified by the availability of gambling taxes, funds, and fees. That's on top of further problems and externalities of gambling, including gambling addiction, organised crime, political corroption, and the like.
