If Michael were smart, he'd allow it. Then when Terri swallows it, he could proclaim it's "A MIRACLE"--never mind that she could have taken foods orally before had they been given--and he'd have an excuse for any improvement in her condition that might occur if she's given therapy.
Michael doesn't want Terri to take Holy Communion, I believe, because it proves she can swallow.
If she can swallow, she won't even need a feeding tube; hence he can't have her killed; hence he doesn't get the $$$$$$; hence if Terri receives therapy, she might tell how she became disabled; hence Michael would not get REVENGE on Terri's dad, for making his life less pleasant.