There are several factors at play here that IT professionals are having to deal with. Some come from the employer but some come from the competition between recruiting firms. As a rule (there are always exceptions) employers view IT as a cost center the same as say custodial services. Employees are not going to be as productive in a filthy workplace same as they will not be as productive if they are doing data processing with a pen and pad or typewriter. Employers understand this fact but resent the salaries that IT commands because only a few understand it. Enter the recruiters. recruiters are trying to fill a position for the employer while staying in the lower range of the salary that the employer has outlined. This is all economics 101.
The more established recruiting firms that are looking for long term placement and low turnover will seek to find the right skill sets first and then find the employee in that group that fits the salary range.
The problem comes from new recruiting firms and recruiters that, in an effort to reduce their own costs and subsequently hold their margins, try to weed out candidates where salary negotiation could take additional time that could be earning them money if they already had someone in the position. The best way to handle companies like this is simply not do business with them because they will NOT be looking out for you.
If you have to deal with them, I recommend that you research the average salary for the position in the geographical market you are in first. Then keep your existing salary confidential but state that you desire is X based upon the average and the amount of experience that you bring to the table.
Keep Up with TechRepublic