Typically the reason a company prefers the signing bonus approach instead of "retro" pay, i think, is because they do not want to have to go back and revise the previous quarters or years financial statements. The signing bonus lets them show the costs at the time that the cost occurred, at contract signing, rather than submit a bunch of revised financials. I am not sure why so many people think it is a concession.

Congrats in the TA,

WG