From what I can determine these sorts of bonds are nothing more than onerous contracts that wouldn't be enforceable if it made it to court.
Furthermore, how can an airline expect to legally enforce a bond when the pilot has gone out and paid for their own type rating for example? We know that certain airlines are renowned for making a tidy profit from type ratings. Whilst there might be some costs in getting the pilot up to standard (say line training) I would argue that these are the normal costs of doing business and any company would surely encounter such costs when a new employee joins. There might be an argument that airlines encounter much higher costs in training up staff (TRE's and TRI's don't come cheap nor do sims) but the bond periods they typically try and enforce are for the most part extreme in the least.
My gut feeling is that the bonds that the locos and others use are there as a means of scare mongering for the uninitiated and of course the more unscrupulous airlines can and will use intimidatory tactics to try and enforce them. For the most part they wouldn't be willing to go all the way and actually front up to court for fear of losing and therefore setting a precedent. Also during the discovery process it wouldn't show them in a very positive light if you were to get your hands on the actual costs of type ratings from outfits like CAE and SAS and demonstrate to the courts the amount of profit made off you even before you have started work! Kind of undermines their case now doesn't it
If it was me I would sign on the dotted line, use a payg mobile phone, work hard and move on when I want to leaving no forwarding address