It is one the interesting parts of the Parliamentary system. Because the Prime Minister holds the Royal perogatives once exercised by the crown, which also includes the ability to dissolve Parliament. Of course, the Prime Minister still has to go to the Queen (or the Governor General as in Canada) and ask for Parliament to be dissolved but the impetus is exercised by the Prime Minister.
Anyway, the snap election is a risky policy, and while in Prime Minister Harper's case the objective is to secure a majority in Parliament, other times it can be driven by purely presentational issues like the new Prime Minister wanting to look like his own man and not one needing to rely on the majority of his predecessor (i.e. Martin & Brown). But while it may seem that the governing party holds all the cards, they're not as much a sure fire scheme of gaining a larger majority than it seems.
As far as I know, in the last 35 years, I don't think any snap-election deliberately called by a Prime Minister either side of the Atlantic has succeeded and has even back fired terribly (i.e Paul Martin). So while I see where you are coming from in theory, you must understand that they've actually got to
win an election first
Also remember that the Opposition, if they are powerful enough can also force an election. Usually, unwritten precedent states that if the government loses a bill in the lower chamber completely then that government has therefore lost the confidence of Parliament and must call an election.
At the end of the day, this will always be a major battleground for the supporters of either a codified or a uncodified system. Those in favour of a codified system will always say that theirs is more efficient and prevents the concentration of power while those in favour of the uncodified version talk of systems which have survived for over 700 years without crisis and thus, why fix what is patently not broken?
I don't really support either myself. I am more in favour of J.J's "grand ruler of Canada" plan where he takes over power in a coup de tat and rules with an iron fist.