U.S. punters: why give the bookies 'the juice'?

Last year, I started following an on-line tipster in Canada.  My reasoning that as much as he thought of himself as a handicapper/sports advisor, my analysis on his betting is that he's simply a gambler. 

His staking I found remarkable.  When betting odds on, he wagers enough to win a point (he says $100.00).  Example:
-128, he wagers $128 to win $100.

Betting underdogs, he wagers a point.  Example:
+155, he wagers $100 to win $155

I remember years ago visiting a U.S. betting site.  Minimum wager was $10.00.  And when I clicked on to place a bet (i.e. -120), my wager became $12.00 and I couldn't make it $10.00.

It seems to me that this is a bookmakers dream to have punters think this way and top up their bets with 'the juice'.  The mentality of a "sure thing" means betting more, almost telling the punter that 'if you want a piece of the winnings, you have to pay for it"

It must totally screw up the perception of value.

Any views on this?