Well, then obviously these companies would not experience price competition from smaller companies when they pass the tax on to the consumer, right? I don't see how that's a problem. If there's a demand for the product, it will be built. It will just be cheapest to build it in the smallest company. Larger companies could still persist at it if their size confers cost savings over the cost of the tax. The point is, rather than having all the financial wizards trying to figure out how to hide money and feed monolithic behemoths, they instead will be working on how to carve themselves up into independent pieces that can be held accountable.
But high unit margins are terrible. First of all, they mean higher consumer prices, which are bad in and of themselves. But also, that unit margin either becomes corporate profit (in which case it makes richer the stockholders of the company, who will, ceteris paribus, be wealthier on average than the consumers paying that cost), or it gets eaten up by the management of the company (in which case it makes richer the executives of the company, who will, ceteris paribus, be wealthier on average than the consumers paying the cost).
It is crazy to suggest that we should pursue a policy that increases consumer prices and redistributes that wealth to the wealthy.
But the point is, you're weakening that. You're making market share less valuable, and unit margin more valuable. So you'll tip the scale more in the direction of high-margin, low-market-share organizations.
Gosh, this forcing fiscal motivators on people sure causes some tricky unintended consequences.
First, when faced with the problem of "big companies buying political influence", it decides to destroy "big companies" rather than "buying political influence". There are many industries and companies where the economics of the business essentially require large companies to be viable. Rather than going after the root of the problem -- buying political influence -- they propose to preemptively punish companies based on a weakly correlated metric without regard for if those companies are engaged in bad behavior. This kind of arbitrarily selective prior restraint is considered an anathema to a free society in every other context and should be here. There is nothing intrinsically bad about big companies and many small-ish companies have out-sized amounts of political influence.
Second, the idea of taxing gross income (and its relative, gross receipts) is not new and many variants of it have been tried in a variety of jurisdictions. Due to the experience of how those implementations actually work out in practice, it is often considered a poor way of structuring taxes. Not only are there many ways to structure around it but it also creates perverse incentives and favors certain kinds of businesses over others. In particular, it tends to put high-growth startups at a disadvantage relative to established companies. In fact, the obvious disadvantage to tech startups is one of the primary considerations for changing these types of taxes in jurisdictions that use them.
In summary, the proposal both ignores the underlying problem and finishes it off by proposing a "solution" that a cursory reading of literature would suggest is obviously defective. It is not clear why we should care about Richard Stallman's opinion on this in any case, since he obviously has very limited familiarity with the subject matter.