That's not entirely true. One can indeed enter more than twice on a visa exemption in any given year, but the passenger must produce (in their own words) "a valid reason" as to why the traveller should be let in. Most countries are already doing this when they see a repeated visitor doing 'visa runs'.

The way I see it is that they're trying to either:
1) keep people who think they can live here on a tourist visa exemption OUT; OR
2) get them to apply for the right visa so that they are compliant with Section (12)(1) and (3) of the Imm. Act of 79', i.e. switching to the DTV or a non-IMM visa. These visas bring extra revenue to the bureau.

In light of this, I don't think they're trying to push legitimate tourists away, and the fact that they're still allowing extensions for the first time but not the second time shows exactly the kind of people the bureau is targeting.