Probably legal (certainly in UK) but to be enforceable the bond must be fair and reasonable in its size and associated terms. The Courts would take a very dim view of any Company which had sought in enforce an unrealistic figure - the Company would have to prove every last penny should the Court so wish it to do so.
Historically very few cases have ever got close to the courtrooms - often an agreement is reached at the last moment, neither side wishing to go into the legal arena.
Probably the greatest threat to the pilot is the potential for the Company he is seeking to leave being in contact with his prospective new employer. The airlines of course expect undying loyalty from their own pilots but at the same time expect a new recruit to jump ship at the first opportunity.
It would be interesting to hear from a qualified lawyer on this one - failing that seek advive from BALPA or IPA/IPF - it is after all a VERY old chestnut...................