(Bloomberg Opinion) -- It’s a good conservative principle that where possible, the government should recover the cost of its services from the people who use them, rather than from taxpayers at large. It’s also pretty uncontroversial that the government must oversee financial markets, to ensure that they are free and fair.

It thus makes sense that the government should charge a user fee for financial transactions. So why has the idea -- as proposed by various politicians, including presidential candidate Bernie Sanders, Senator Brian Schatz and Representative Peter DeFazio -- encountered so much opposition?

It’s not as if this were radical socialism. Hong Kong, perennially rated the world’s freest economy by the conservative Heritage Foundation, has had a 0.1% tax on financial transactions for years. The levy has had no discernible negative effect on its economy, though it might be responsible for a relative lack of high-frequency trading. Many other countries have financial transaction taxes, including the U.K., Switzerland and Taiwan.

Opponents of the tax offer two main arguments. First, they say the burden will fall mostly on small investors. Second, they say it will undermine the ease of buying and selling -- or liquidity -- that makes U.S. markets so attractive, and impair those markets’ ability to determine the proper prices of securities.

Let’s examine the first claim. The idea is that regular folks mostly invest through mutual funds, which trade a lot and hence will get hit hard by the tax. Specifically, in a letter to legislators, the Investment Company Institute estimated that the tax would impose a 60% average cost increase on investors in equity index funds.

That calculation is specious at best. It implies, for example, that the typical investor holds an index fund for less than six years. According to the ICI, this is based on purchase and sale data from 2018 -- a year in which the funds experienced large redemptions from retiring baby boomers and vast inflows from investors looking to reduce their fees.

Actually, people who invest in index funds for their retirement tend to be long-term buy-and-hold investors. An investment at age 35 might be withdrawn at 65, which suggests a holding period of about 30 years -- or even longer if the money is funding a bequest. Given that horizon, the average tax per year is less than one-hundredth of one percent, which would increase the typical index-fund fee by only 8%. The tax on the funds’ own trading might add a little to this, but not much.

What about liquidity? True, the tax would put the brakes on the high-frequency outfits, proprietary traders and hedge funds whose chief mission is to profit by launching swarms of lightning trades at the expense of slower-moving “whales” such as mutual and pension funds. But that should be good for retail investors, on whose behalf the latter institutions are supposed to be investing. It should also be good for markets more broadly, reducing the threat that high-frequency algorithms gone wrong will cause a systemic crisis -- as they almost did in the “flash crash” of 2010.

All told, the benefits of a tax on trading far outweigh the costs. It would generate much-needed revenue. It would favor longer-term investors over speculators. It would put a little useful resistance into the financial system, preventing it from overheating or spinning out of control. And it would pass the fairness test by placing the cost of running the system on the people who use it most.

So why do the tax’s opponents -- who are mostly from the investment management industry -- make mountains out of molehills, exaggerating its burdens and dangers? It’s hard not to conclude that they’re really trying to protect their already ample profits against any and all constraints. That’s completely understandable, but a terrible foundation for making policy.

To contact the author of this story: Michael Edesess at medesess@gmail.com

To contact the editor responsible for this story: Mark Whitehouse at mwhitehouse1@bloomberg.net

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Michael Edesess is chief investment strategist with the mobile financial-planning software company Plynty and a research associate at the EDHEC-Risk Institute.

©2019 Bloomberg L.P.