Might this unfairly subsidise companies paying higher salaries? So Google and their employees would get more money from the government to support their pregnant employees than Joes Pizza Company? Why?

Companies that pay higher salaries by their choice incur higher payroll costs. That’s just how that works. Trying to engineer the system to even that out somehow is just introducing bureaucracy and costs into the system. The only case I do have concerns about is small employers who by chance might get hit with the costs of a large proportion of their staff taking maternity/paternity leave at the same time. That could be crippling and possibly there should be some special programme to help those companies out.