I really don't see the big deal here! Whether you are bonded for $50,000 for 3 years or you are bonded with a salary reduction the difference is quite negligible in the long term if you intend on doing the honorable thing and seeing out your bond period. The numbers jumping ship are huge so the company has to do something. After the 18 months of payments your bond will of reduced to the same amount as you have paid so if you jump ship at that time your all square.
Standard your figures are wrong with regards to your returns on $25,000. @ 6% over 3 years. Your investment assumes you have $25,000 to invest from day 1 in Doha, which you don't. You would be saving $1,389 per month for 18 months which would give you $26,224 @ 6%. Which would then grow at 6% for a further 18 Month with no extra contributions to give $28,687. Based on interest paid monthly. So your gain is $3,687. So divide that by 36 months and your actual reduction in salary could be viewed as only $102.42 per month. Which is about 1 night a month at the pub. Which you won't really notice and the forced savings gives you a nice cheque at the end of 3 years which you probably wouldn't of saved had you been given it on a monthly basis anyway. So one view could be that you'll be better off!! Just my thoughts though!