I strongly suspect that it's not the $5m/yr these companies are spending that's doing it; there's basically three other camps who push (directly or indirectly) for this:

* Special-interest groups whose preferential tax treatment might be threatened if there's a push to simplify the tax code (as having the government do the taxes for you kind of requires the taxes be simpler to do so).

* Ideologues who hate government spending but don't think that tax cuts count as spending.

* Anti-tax crusaders who want to make filing taxes painful so there's more grassroots support for cutting taxes. (Think Grover Norquist here).