I think it´s rather normal procedure for multi-band gigs. The headliner is expected to be the loudest (hence dB-limits for supporting acts) and best sounding (usually due o their own dedicated engineer, who knows the band, their sound and material well, and has done many gigs with them).
Support act dB´s are limited for audience protection (if it were 100dB+ for the whole evening, everybody´s hearing would be damaged) and "dramaturgy" (the headliner is THE HIGH POINT for the evening).
I´ve experienced following (as a FOH engineer):
-The headliner refused to share input channels (them taking up 28 channels of the 33 available, and the band I was doing required 17 minimum...). Later, with pressure from house engineer, they let me use some of their channels, provided I didn´t touch their settings...
-The aforementioned band, a trio, had scattered their backline on the very big stage, and thus we had difficulties loading our stuff in. We asked if we could slightly move one of the keyboards to ease our set-up. The answer was: "ummm... no."...
-The headliner taking over 3,5 hours to complete their soundcheck (11 input channels, and using 13 of the available 16 compressor channels!), thus cutting our setup and soundcheck time into a comfy 25 min... They also reserved half of the effects processors for "their use only". BTW, on this night, the headliner was so loud that most of the audience walked out of the hall. Nobody´s ears could take it.
-Many cases of dBSPL limits over 10dB less than the headliner. As many cases of lighting restrictions ("no strobes, no blinders, no moving heads" etc...
Nowadays, the band does more headlining than support gigs, so I´m happier...