I can understand it from both sides.
Previously Agency got more because they didn’t work all year, you worked The Season, you got paid more because work (or the lack of it) was more risky. The same way as you get a lower per day price for a hire car than you do for one days hire (if you see what I mean!) then Salary people got less but were guaranteed money.
Agency guys turn up, swan about on your system earning a packet more than you but not knowing the system? yes, annoying. However you also get Salary guys who aren’t clever enough to go Agency (cos they would starve!) who are quite happy to sit back and get paid their salary whether they work or not or whether they do a good job or not.
Theres good guys and bad guys on both sides 😀