Actually, it seems to me that systems have evolved differently. In a system like that of the US, the legislator's staying in her seat is *not* directly tied to how the legislature votes on any particular issue. In Brit-style parliamentary democracies, lose the wrong vote in the parliament and it's time for a new election. That difference has the effect of making it *easier* to "break party lines" in the US and "vote your conscience" (or your personal gain, if you're Machiavellian enough).

Philosophy can be made out of anything. Or less -- Jerry A. Fodor